jextract/jni: Implement generic equals method

[sidepelican]

In Java, every Object has an equals method.
Conceptually, Swift's Equatable protocol serves the exact same purpose as Java's equals.

This PR implements a generic equals method, enabling all JNISwiftInstance types to leverage Swift's Equatable comparison directly through the standard Java equals method.

[ktoso]

Neat! I wonder if we can nicely do hash code as well; otherwise we're breaking the contract of always having to implement "both"

[sidepelican]

I forgot to consider hashCode.
It's a bit challenging (Only Equatable types cannot provide hashCode)

[sidepelican]

Implemented hashCode().
I made a compromise regarding types that only implement Equatable.

[ktoso]

Looks good, let’s add a test putting swift values into a java set maybe or
Something like that?