Investigate whether manually written hashcode() methods are still needed

[alicejli]

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.