fix: Handle external file groups in ExternalFilePathUtil

[vinishjail97]

Describe the issue this Pull Request addresses

PR adds support for external base files with file group prefixes, allowing external files to be organized in subdirectories within partitions. (Paimon for example organizes parquet files into partitions and each partition has multiple buckets).
The implementation consolidates all external file handling logic in ExternalFilePathUtil for better maintainability.

Summary and Changelog

Support for external files with file group prefix format: <fileName>_<commitTime>_fg%3D<prefix>_hudiext
Consolidated external file parsing logic in ExternalFilePathUtil.parseFileIdAndCommitTimeFromExternalFile()
Added comprehensive test suite for ExternalFilePathUtil.

Impact

Enables hudi metadata generation for external files that are organized in subdirectories within partitions, useful for generating hudi metadata for Paimon tables.

Risk Level

Low. The changes maintain backward compatibility with existing external files (without prefix) while adding support for the new format.

Documentation Update

None. Updated java doc for ExternalFilePathUtil.

Contributor's checklist

• Read through contributor's guide
• Enough context is provided in the sections above
• Adequate tests were added if applicable

[vinishjail97]

Error:  Plugin org.jacoco:jacoco-maven-plugin:0.8.12 or one of its dependencies could not be resolved:
Error:  	The following artifacts could not be resolved: org.jacoco:jacoco-maven-plugin:pom:0.8.12 (absent): Could not transfer artifact org.jacoco:jacoco-maven-plugin:pom:0.8.12 from/to central (https://repo.maven.apache.org/maven2): repo.maven.apache.org: Temporary failure in name resolution
Error:  -> [Help 1]
org.apache.maven.plugin.PluginResolutionException: Plugin org.jacoco:jacoco-maven-plugin:0.8.12 or one of its dependencies could not be resolved:
	The following artifacts could not be resolved: org.jacoco:jacoco-maven-plugin:pom:0.8.12 (absent): Could not transfer artifact org.jacoco:jacoco-maven-plugin:pom:0.8.12 from/to central (https://repo.maven.apache.org/maven2): repo.maven.apache.org: Temporary failure in name resolution

There are temporary dependency resolution errors from maven.

[hudi-bot]

CI report:

• d63d8ef Azure: SUCCESS

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