Audit serialization trait implementations

[0xZaddyy]

Summary

This PR Audits serialization trait implementations across the codebase to prevent unintentional API exposure and reduce dependencies. This change makes serde serialization optional behind a feature flag rather than mandatory.

Closes #1147

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[coveralls]

Pull Request Test Coverage Report for Build 20133889849

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.176%

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Totals
Change from base Build 20109603088:	0.0%
Covered Lines:	9665
Relevant Lines:	11620

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💛 - Coveralls

[nothingmuch]

I don't know about the serde usage in the ffi stuff and how appropriate it is to remove it for those types

Only commenting about the removal and feature gating in the payjoin crate: i don't think a feature flag for this is desirable because:

• it's one more feature flag, or 2x the feature flag combinations, which are already intractable for us to test
• serialization is required as part of persistence for correct implementation of the protocol, and not all struct fields are pub

so with the feature disabled the crate couldn't be used to write a safe implementation

so i think this dependence on serde in our public api is actually desirable and intended

[spacebear21]

Removing the De/Serialize from payjoin-ffi seems appropriate, they don't do anything there and were probably just copied over unintentionally. Agreed about keeping serde tightly coupled with rust-payjoin for persistence.