[MNG-8768] Add executable() function for conditional profile activation based on PATH

[Hiteshsai007]

Implements MNG-8768 (GitHub #10115): adds an executable(name_or_path) function to Maven's condition-based profile activation DSL.

The function evaluates to true if the given executable name can be found in the system PATH, or if an absolute/relative path points directly to an executable file.

Use-case (from the issue): auto-detect the presence of x86_64-linux-musl-gcc when building GraalVM native images with the native-maven-plugin, and conditionally add --static --libc=musl to the build options only when the compiler wrapper is available on the current machine.

Example pom.xml usage:

executable('x86_64-linux-musl-gcc')

Implementation details:

• ConditionFunctions.executable() delegates to new ExecutableFinder helper
• ExecutableFinder splits the env.PATH system property and probes each directory; on Windows it also tries .exe / .cmd / .bat / .com extensions
• Absolute/relative paths (containing a path separator) are checked directly without PATH traversal
• PATH is sourced from ProfileActivationContext.getSystemProperty('env.PATH') (Maven's normalised env var key), with a System.getenv('PATH') fallback

Tests:

• 5 new end-to-end tests in ConditionProfileActivatorTest (MNG-8768)
• 12 new unit tests in ExecutableFinderTest covering PATH search, Windows extension probing, absolute paths, non-executable files, and empty PATH

Following this checklist to help us incorporate your
contribution quickly and easily:

• Your pull request should address just one issue, without pulling in other changes.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Each commit in the pull request should have a meaningful subject line and body.
  Note that commits might be squashed by a maintainer on merge.
• Write unit tests that match behavioral changes, where the tests fail if the changes to the runtime are not applied.
  This may not always be possible but is a best-practice.
• Run mvn verify to make sure basic checks pass.
  A more thorough check will be performed on your pull request automatically.
• You have run the Core IT successfully.

If your pull request is about ~20 lines of code you don't need to sign an
Individual Contributor License Agreement if you are unsure
please ask on the developers list.

To make clear that you license your contribution under
the Apache License Version 2.0, January 2004
you have to acknowledge this by using the following check-box.

• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004
• In any other case, please file an Apache Individual Contributor License Agreement.