[sdk-platform-java] Investigate whether manually written hashcode() methods are still needed

There are several manually written `hashcode()` methods in the generator:

1) TypeNode: https://github.com/googleapis/sdk-platform-java/blob/v2.34.0/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/TypeNode.java#L256

2) Field: https://github.com/googleapis/sdk-platform-java/blob/v2.34.0/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Field.java#L109

3) ScopeNode: https://github.com/googleapis/sdk-platform-java/blob/v2.34.0/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ScopeNode.java#L67

4) VaporReference: https://github.com/googleapis/sdk-platform-java/blob/v2.34.0/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/VaporReference.java#L125

5) ResourceName: https://github.com/googleapis/sdk-platform-java/blob/v2.34.0/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/ResourceName.java#L147

6) MethodArgument: https://github.com/googleapis/sdk-platform-java/blob/v2.34.0/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/MethodArgument.java#L63

7) https://github.com/googleapis/sdk-platform-java/blob/v2.34.0/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/ResourceReference.java#L44

This is a tracking bug to investigate if they are needed or if we can remove this in favor in the built-in `hashcode()` and `equals()` methods. If they are still needed, the reasoning should be documented in the comments.