relay: ensure API types are public

[bendk]

I'm working on a new UniFFI parser
(mozilla/uniffi-rs#2841) and it currently requires that types used in the exported functions are publicly available from other crates.

I could maybe rework the parser to handle this another way, but this feels cleaner to me anyways. It feels weird if a type can be used by foreign languages but not other Rust crates.

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