[filesystem] Isolate Jindo OSS presigning dependencies

[JunRuiLee]

Purpose

Prevent the OSS SDK dependency closure bundled in paimon-jindo from shadowing libraries supplied by an embedding application. For example, the bundled Gson 2.8.6 can mask a newer Gson used by Doris, depending on classpath order.

Reuse the same PluginLoader and private-directory classloader isolation mechanism already used by paimon-oss, rather than maintaining a package-relocation list. This applies to the OSS presigning implementation and its dependency closure; it does not change the Jindo/Hadoop filesystem loading path or introduce a separate implementation Maven module.

Specifically:

• Load the OSS presigning implementation, SDK dependencies, helper, and resources from paimon-plugin-jindo-oss/ instead of exposing them at the JAR root.
• Keep the Jindo/Hadoop boundary unchanged. Share Paimon types and logging APIs with the host, and retain the existing Java 11 JAXB multi-release entries.
• Initialize the presigner lazily and restore the thread context classloader after initialization, signing, and shutdown.
• Add packaged-artifact regression tests without introducing another Maven module or modifying the common classloader implementation.

This PR targets master; a separate release-2.1 backport can follow after the mainline change is accepted.

Tests

Ran the following on JDK 8 and JDK 11 against this master-based commit:

mvn -B -ntp -pl paimon-filesystems/paimon-jindo -am \
  -Dmaven.jar.forceCreation=true \
  '-Dtest=JindoFileIOTest#testPlugin*+testCreateBlob*+testDisable*+testKeep*' \
  -DwildcardSuites=none -DfailIfNoTests=false verify

Both runs pass: 4 selected unit tests and 4 packaged-artifact isolation tests per JDK, with Checkstyle, Spotless, and Enforcer enabled.

The artifact tests cover root/private JAR contents and SPI discovery, a minimal host classpath, conflicting dependencies before and after the plugin JAR, independent Gson serialization, private OSS resources, shared logging classes, classloader-safe reflection, presigning, and context-classloader restoration.

The module Apache RAT check also passes (15 approved files, 0 unapproved), excluding only the generated dependency-reduced-pom.xml. git diff --check passes.

Local validation does not include a live Doris/OSS deployment. The existing native-filesystem fallback test was not selected because the local macOS/arm64 environment cannot run the Jindo native filesystem; the test itself remains enabled for supported environments.

[JunRuiLee]

This was found while integrating Paimon 2.1 into Doris: the OSS SDK bundled at the root of
paimon-jindo shadows libraries the host provides — the bundled Gson 2.8.6 masks the newer
Gson used by Doris, depending on classpath order.

The fix moves it into a private plugin directory via the existing PluginLoader, same as
paimon-oss. No new Maven module, no change to the shared classloader.