java: add generate command

[julieqiu]

Add generate command for Java.

[zhumin8]

update: started testing code in https://github.com/zhumin8/librarian/tree/test-java
Testing in google-cloud-java with librarian.yaml as

language: java
sources:
  googleapis:
    # Path to local googleapis checkout
    dir: ../../googleapis

default:
  output: .
  java:
    generator_jar: ./tools/gapic-generator-java.jar
    grpc_plugin: ./tools/protoc-gen-java_grpc
    formatter_jar: ./tools/google-java-format.jar

libraries:
  - name: secretmanager
    output: ./java-secretmanager
    apis:
      - path: google/cloud/secretmanager/v1
      - path: google/cloud/secretmanager/v1beta2
      - path: google/cloud/secrets/v1beta1
  - name: vision
    output: ./java-vision
    keep:
      - proto-google-cloud-vision-v1/src/main/java/com/google/cloud/vision/v1/ImageName.java
      - google-cloud-vision/src/test/java/com/google/cloud/vision/it/ITSystemTest.java
      - google-cloud-vision/src/test/resources/city.jpg
      - google-cloud-vision/src/test/resources/face_no_surprise.jpg
      - google-cloud-vision/src/test/resources/landmark.jpg
      - google-cloud-vision/src/test/resources/logos.png
      - google-cloud-vision/src/test/resources/puppies.jpg
      - google-cloud-vision/src/test/resources/text.jpg
      - google-cloud-vision/src/test/resources/wakeupcat.jpg
    apis:
      - path: google/cloud/vision/v1
      - path: google/cloud/vision/v1p1beta1
      - path: google/cloud/vision/v1p2beta1
      - path: google/cloud/vision/v1p3beta1
      - path: google/cloud/vision/v1p4beta1

Currently can generate bare-bone libraries with diffs mostly due to:

• pom generation not added
• readme and repometadata generation not added
• header fix for grpc and proto modules not added
• configs not added/respected, e.g. for retry, batching ...

Will add header fix logic and start breaking down and raising PRs to main branch.