[pigeon] Implement equals for Java data classes

[stuartmorgan]

Adds implementations of equals and hashCode to Java data classes. This is frequently useful for native unit tests of plugins using Pigeon (e.g., when using a mock FlutterApi implementation to check that the expected call is being made with the right arguments).

Part of flutter/flutter#118087

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[stuartmorgan]

This change is because an inherent limitation in Java equals implementations is that List equality only works if each element in the list has a useful implementation of equals, and (as demonstrated in the new generator code) arrays don't have that. So having the generic list's value be a nested array makes equality not work. Fixing that would require a bunch of special code generation—we would have to do custom equality checks of all collections to call out to Arrays.equals for certain types—for a case that I'm skeptical will ever matter in practice, and is a common behavior of the language anyway.

Since the specific type here didn't actually matter, I opted for just changing to something with a useful equals so that the test will pass.

[tarrinneal]

Looks like you need to gen/format a file

[tarrinneal]

a questionable nit, make the gen code cleaner, but the generator messier?

[tarrinneal]

Should we make this a helper?

[stuartmorgan]

Doing it inline like this seems to be the stand way I've seen it in other Java code. We'd still need the assignment, so we'd be adding a function call just to abstract away 31 * __pigeon_result + which seems like a lot of overhead.