[filesystem] Add JuiceFS filesystem support

[weizuo93]

Purpose

Linked issue: close #3866

This PR adds support for JuiceFS — a widely-adopted open-source distributed file system that is built on top of object storage and exposes a Hadoop-compatible Java SDK. With this change, users can point remote.data.dir at a jfs:// volume and store Fluss snapshots (Primary-Key Tables) and tiered log segments (Log Tables) on their existing JuiceFS deployments

Brief change log

• Add a new Maven module fluss-filesystems/fluss-fs-juicefs, registered as a child of fluss-filesystems.
• Implement JuiceFsPlugin implements FileSystemPlugin:
  • Registers the jfs:// URI scheme.
  • Loads io.juicefs.JuiceFileSystem reflectively via org.apache.hadoop.fs.FileSystem.newInstance, so Fluss has no hard compile-time dependency on io.juicefs.* classes.
  • Forwards any Fluss configuration entry whose key starts with fs.jfs. or juicefs. to the underlying Hadoop Configuration.
  • Injects sensible defaults when not set by the user:
    • fs.jfs.impl = io.juicefs.JuiceFileSystem
    • fs.jfs.impl.disable.cache = false
• Implement JuiceFsFileSystem extends HadoopFileSystem:
  • Wraps the JuiceFS Hadoop FileSystem.
  • Returns an empty placeholder security token from obtainSecurityToken(), because JuiceFS handles authentication locally on each node against its metadata engine and backing object storage — no STS / delegation-token exchange is required.
• SPI registration via META-INF/services/org.apache.fluss.fs.FileSystemPlugin.
• Shading / packaging consistent with sibling filesystem plugins; depends on fluss-fs-hadoop-shaded, fluss-fs-hadoop, and io.juicefs:juicefs-hadoop:1.4.0.
• Documentation:
  • New page website/docs/maintenance/tiered-storage/filesystems/juicefs.md.
  • Add JuiceFS to the overview.md list of pluggable filesystems.

Tests

Unit tests added in JuiceFsPluginTest:

• testScheme — verifies the plugin registers the jfs scheme.
• testHadoopConfigPropagation — verifies fs.jfs.* and juicefs.* entries are forwarded to the Hadoop Configuration, and that unrelated keys are not forwarded.
• testApplyJuiceFsDefaultsInjectsImpl — verifies default injection of fs.jfs.impl and fs.jfs.impl.disable.cache.
• testApplyJuiceFsDefaultsDoesNotOverrideUserValues — verifies user-supplied values take precedence over the injected defaults.
• testGetHadoopConfigurationWithNullFlussConfig — null-safety.

Run locally with:

mvn test -Dtest=JuiceFsPluginTest -DfailIfNoTests=false -pl fluss-filesystems/fluss-fs-juicefs -am

API and Format

No changes to public APIs, wire format, or on-disk / storage format. The change is purely additive — a new implementation of the existing FileSystemPlugin SPI.

Documentation

• New page: website/docs/maintenance/tiered-storage/filesystems/juicefs.md — installation, prerequisite (JuiceFS volume creation), required / optional server.yaml configuration, authentication model, and pointers to the JuiceFS Hadoop Java SDK documentation for advanced tuning.
• Updated website/docs/maintenance/tiered-storage/filesystems/overview.md to include JuiceFS in the intro list and add a bullet under the "Pluggable File Systems" section.