Generate method overloads when a resource field has multiple resource names configured

[yihanzhen]

• When a field has child_type reference and the referenced resource has multiple patterns, allow this field to be associated with multiple resources if all these resources combined have exactly all the parent patterns of the resource referenced by child_type.
• Generate method overloads if the field with multiple resource names are in a flattening config.
• If there are multiple fields with multiple resource names, generate all possible combinations.
• Always generate an overload with all resource names treated as strings.
• In Java, repeated fields with resource name configs are always treated as List<String> to avoid multiple methods having the same signature after type erasure.
• The * annotation is not yet supported.

See changes to ArchiveBooks, LongRunningArchiveBooks and StreamingArchiveBooks for testing generating overloads for unary, longrunning and streaming methods.

See changes to MoveBooks for testing generating all the combinations.

[codecov]

Codecov Report

Merging #3081 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #3081   +/-   ##
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  Coverage     87.03%   87.03%           
  Complexity     6000     6000           
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  Files           493      493           
  Lines         23869    23869           
  Branches       2584     2584           
=========================================
  Hits          20775    20775           
  Misses         2239     2239           
  Partials        855      855

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[chrisdunelm]

LGTM from the generated code point of view. Just one question.

[chrisdunelm]

Is it correct that a required resource-name is allowed to be null?

[yihanzhen]

This has been how java delegates the overload with resource name to the non-flattened method.

For example: https://github.com/googleapis/java-tasks/blob/master/google-cloud-tasks/src/main/java/com/google/cloud/tasks/v2/CloudTasksClient.java#L1532-L1540

Actually I think protobuf Java does not allow setting null values on any field, so the code will still fail on the client side. I guess we just pass the error handling to protobuf :)

[vam-google]

LGTM (based on our in-person conversation)