java(serviceconfig): populate sdk.yaml with fields needed for Java new libraries

Java new libraries grabs a few additonal fields from service config today, like rest_reference_documentation_uri, proto_reference_documentation_uri. These should be added to populateFromServiceConfig, and write to sdk.yaml.

for example: https://github.com/googleapis/google-cloud-java/blob/eb412a1c5c13700ac14c11754f668808035d7679/generation_config.yaml#L68-L76

related issue: #5585

Find the serviceconfig implementation containing populateFromServiceConfig and trace how its values are written to sdk.yaml. Compare the Java generation_config.yaml example linked in the issue, including rest_reference_documentation_uri and proto_reference_documentation_uri. Done means the required service-config fields are populated and emitted in sdk.yaml for Java libraries, with the relevant tests passing.

Written by the indexing model from the issue text.