Gradle scripts changes to enable plugin signing

[ikopylov]

Splits the buildPlugin Gradle task into a two-phase build process to support external code signing of plugin artifacts
(DLLs and JARs) between the build stages.

New Gradle tasks:

• preparePluginForSigning - compiles all artifacts, copies them to a staging directory (build/plugin-staging/), and
  generates a files-to-sign.txt manifest listing files that need signing (our own JARs and DLLs, excluding third-party
  dependencies)
• assemblePlugin - packages the staged (optionally signed) files into the final plugin ZIP and creates the NuGet
  package

How it works:

• For local development, ./gradlew buildPlugin continues to work as before - it runs both phases sequentially with no
  signing step in between
• For CI/CD with signing, the build can be split into separate Gradle invocations with an external signing step in the
  middle:
  ./gradlew preparePluginForSigning → sign server signs files → ./gradlew assemblePlugin