refact: merge pd into hugegraph

[VGalaxies]

Purpose of the PR

subtask of apache#2265

Merge the internal version of pd into hugegraph.

Main Changes

1. Merge the internal version of pd into hugegraph-pd module.
2. Modified some pom files under the hugegraph-pd module so that it can be built normally under certain conditions.
3. Clean some comments in hugegraph-pd.
4. Add apache-rat-plugin for hugegraph-pd and add some license headers.
5. (TODO) potential code cleanup or style issues

NOTE:

1. Due to the modification of <parent> in the root pom of hugegraph-pd from spring-boot-starter-parent to hugegraph, it is necessary to explicitly specify the version of some dependencies for the hugegraph-pd submodules.
2. The hugegraph-pd module incorporates features that are not supported in Java 8, such as the var keyword. To ensure compatibility with Java 8, subsequent PRs could consider adjusting the compilation version based on whether the user opts to use hugegraph-pd and hugegraph-store.

Verifying these changes

• Trivial rework / code cleanup without any test coverage. (No Need)
• Already covered by existing tests, such as (please modify tests here).
• Need tests and can be verified as follows:
  • subsequent PRs will merge store and more adaptation work needs to be done

Does this PR potentially affect the following parts?

• Nope
• Dependencies (add/update license info)
• Modify configurations
• The public API
• Other affects (breaking changes)

Documentation Status

• Doc - TODO
• Doc - Done
• Doc - No Need