Add HttpClient5 sampler implementation with HTTP/2 support

[andreaslind01]

Description

This PR adds a new HTTP sampler implementation, HttpClient5, based on Apache HttpComponents HttpClient 5.x, and introduces a configurable HTTP Version setting (HTTP/1.1 / HTTP/2) for both the HttpClient5 and the Java implementation.

Main changes:

• New HTTPHC5Impl (HTTPSamplerFactory.IMPL_HTTP_CLIENT5, selectable as HttpClient5 in the GUI and in JMX files):
  • Classic (blocking) client for HTTP/1.1 and an async H2 client for HTTP/2, selected via HttpVersionPolicy (FORCE_HTTP_1 / NEGOTIATE), with automatic fallback to HTTP/1.1 when the server does not offer h2 via ALPN.
  • Per-thread client caching keyed by target/proxy/timeouts/local address/version policy.
  • Support for connect & response timeouts, proxies (incl. proxy authentication), AuthManager (BASIC/DIGEST, pre-emptive BASIC), CacheManager (conditional requests via If-Modified-Since / If-None-Match), CookieManager, DNSCacheManager, response decompression (gzip/deflate/brotli), and retry handling.
  • Correct population of SampleResult metrics: sentBytes, connectTime (measured for both HTTP/1.1 and HTTP/2, including TLS), latency, headers and response code/message.
• HTTPJavaImpl: HTTP/2 support via the JDK java.net.http.HttpClient when HTTP/2 is selected, including caching, proxies, user authentication, sentBytes accounting, connect-time measurement, reason-phrase derivation (HTTP/2 has no reason phrase) and preservation of Authorization / Proxy-Authorization headers.
• New sampler property HTTPSampler.httpVersion (HTTPSamplerBaseSchema.httpVersion, getter/setter on HTTPSamplerBase) with a new combo box in HTTP Request and HTTP Request Defaults (http_version resource key added to all messages_*.properties).
• CacheManager: new overloads for HC5 (ClassicHttpRequest / ClassicHttpResponse / org.apache.hc.core5.http.Header[]) and for the JDK java.net.http.HttpResponse.
• Property httpclient.version re-purposed as the default HTTP version (HTTP/1.1 | HTTP/2) used when the sampler's HTTP Version field is empty.
• Dependencies: httpclient5 and httpcore5 added to src/protocol/http and to the third-party BOM (httpcore5:5.3.4).
• Documentation updated: component_reference.xml, properties_reference.xml, get-started.xml, bin/jmeter.properties.

Motivation and Context

JMeter's HTTP samplers currently only support HTTP/1.1: the HttpClient4 implementation is built on the HttpComponents 4.x line, which will not receive HTTP/2 support, and the Java implementation used the legacy HttpURLConnection. Modern web applications and APIs are increasingly served over HTTP/2, so load tests against them either could not be executed at all or did not represent realistic client behaviour (multiplexing, HPACK header compression, single connection per origin).

This change gives users a supported migration path to HttpComponents 5.x and makes it possible to run load tests over HTTP/2 — either with the fully featured HttpClient5 implementation or, for lightweight scenarios, with the JDK client in the Java implementation. Existing test plans are unaffected: HttpClient4 remains the default and an empty HTTP Version falls back to the previous HTTP/1.1 behaviour.

Fixes:

• HTTP Request: Migrate to HTTPClient 5 APIs #5081
• Support HTTP/2 protocol #4023

How Has This Been Tested?

• New unit/integration tests (34 tests, all green):
  • TestHTTPHC5Features (16 tests): version selection and precedence (sampler value vs. httpclient.version vs. unsupported value), HTTP/2 usage, fallback to HTTP/1.1 when the server does not support h2, HTTP/2 via proxy, sentBytes for GET/POST, conditional requests through CacheManager, BASIC credentials from AuthManager, proxy authentication, and connectTime for HTTP/1.1 and HTTP/2.
  • TestHTTPJavaFeatures (~16 tests): version selection, HTTP/2 requests (incl. via proxy), response message / reason-phrase handling for HTTP/2, sentBytes for GET/POST in both versions, Authorization header from the HeaderManager, and connectTime for HTTP/1.1, HTTP/2 plaintext and HTTP/2 over TLS.
  • TestHTTPSamplerFactory: creation and lookup of the new HttpClient5 implementation, plus the unchanged behaviour for the existing aliases.
  • The tests run against locally started embedded HTTP/1.1, HTTP/2 (h2c and h2 over TLS) and proxy servers, so no external services are required.
• ./gradlew classes style — compiles cleanly and reports no style/checkstyle/autostyle violations.
• The existing src:protocol:http test suite (including JMeterTest, extended by httpVersion in the ignored-properties list) still passes.
• Manual verification in the JMeter GUI: the new HTTP Version combo box in HTTP Request and HTTP Request Defaults is saved/restored correctly in JMX files, and requests against an HTTP/2 endpoint are reported as HTTP/2 in the View Results Tree.
• Test environment: Windows, JDK 21 toolchain, Gradle build JMeter 6.0.0-SNAPSHOT.

Screenshots (if appropriate):

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.