Allow mutable records

[skeet70]

This will make it so custom types can be fully tested with mutation. To match what gets generated for records by Java itself, getters are just name(), not getName(). Setters are setName(Type name).

[github-actions]

File	Coverage	Lines
All files	65%	65%	✅
src/gen_java/mod.rs	71%	71%	✅

Minimum allowed coverage is 0%

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