feat: Add macros to customize JavaBuilder and Error Prone checkers

[JonathanPerry651]

Description

Exposes two new Starlark macros to support pluggable Error Prone configuration and custom Javac checker injection:

1. errorprone_with_custom_plugins: Merges a base Error Prone compiler core library with extra custom plugin/checker libraries.
2. default_javabuilder: Assembles a custom JavaBuilder deploy jar by combining the core javabuilder_without_errorprone binary with the customized Error Prone target.

Motivation

Part of the JavaBuilder pluggability refactoring to allow users to build and run custom Error Prone plugins in their java toolchains.

This PR provides the Starlark macro implementation and defaults for Bazel Issue #29910.

Build API Changes

Yes

1. Has this been discussed in a design doc or issue? Yes, discussed in Bazel Issue #29910.
2. Is the change backward compatible? Yes.
3. If it's a breaking change, what is the migration plan? N/A.

Checklist

• I have added tests for the new use cases (if any).
• I have updated the documentation (if applicable).

Release Notes

RELNOTES: None

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