wiremock-grpc-extension for test scope only

[jayeshmann]

same as demonstrated
https://github.com/wiremock/wiremock-grpc-demos/blob/main/java/build.gradle

keeping it in implementation tree can cause issues with grpc server

References

• https://github.com/wiremock/wiremock-grpc-demos/blob/main/java/build.gradle

Submitter checklist

• The PR request is well described and justified, including the body and the references
• The PR title represents the desired changelog entry
• If the change against WireMock 2 functionality (incompatible with WireMock 3),
  it is submitted against the 2.x branch
• The repository's code style is followed (see the contributing guide)

Details: Contributor Guide