Add Java Platform Module System (JPMS) Support

Add support for running functions on the module path (--module-path) instead of only the classpath (--classpath) to enable full Java Platform Module System (JPMS) features in the Functions Framework.

Currently, the Functions Framework for Java only supports classpath-based execution, which prevents developers from utilizing JPMS features at runtime even when their code is properly modularized with module-info.java files.

Modern Java applications using JPMS face the following issues:
1. module-info.java files are ignored at runtime (treated as regular classes on classpath)
2. provides...with service provider declarations don't work, requiring fallback to legacy META-INF/services files
3. Module encapsulation and strong accessibility rules are not enforced
4. No module-level dependency management or version conflict resolution
5. Cannot leverage jlink to create optimized custom runtime images with only required modules, resulting in larger deployment artifacts

This feature request requires changes across three layers:

1. Local Development (Invoker CLI) - Add --module-path support
2. Build Tooling (Maven/Gradle plugins) - Add module-path configuration (right now classpath is hardcoded [here](https://github.com/GoogleCloudPlatform/functions-framework-java/blob/de202439e223dcb48e3a3319e4c0d57adc450ec8/function-maven-plugin/src/main/java/com/google/cloud/functions/plugin/RunFunction.java#L68C5-L68C58)
3. Cloud Runtime Infrastructure - Update Google Cloud Functions runtime to support modular execution

Start with function-maven-plugin/src/main/java/com/google/cloud/functions/plugin/RunFunction.java at the hardcoded classpath configuration. Trace how the Invoker CLI and Maven/Gradle plugins launch functions, then determine how module-path configuration must reach the cloud runtime. Done requires coordinated support across local invocation, build tooling, and cloud execution.

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