Make "mix" tracing addresses customizable (GEN-892)

[sritchie]

Still needs a docs update and tests.

[linear]

GEN-892 mix addresses should be parameters, not hardcoded

[gitstream-cm]

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src/genjax/_src/core/generative/generative_function.py

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src/genjax/_src/generative_functions/combinators/mixture.py

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[MathieuHuot]

@sritchie great idea! I think it should be customisable as you propose in this PR, and maybe the default should be to add no extra outer address (as if we were using .inline or as Fabian found it we can currently write @ () and get the same effect).
And maybe we could have a little checker at trace time that checks whether addresses are the same in both branches and return the difference as a warning if not!