chore: remove async-trait

[MujkicA]

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• I have linked to any relevant issues.
• I have updated the documentation.
• I have added tests that prove my fix is effective or that my feature works.
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[iqdecay]

Maybe add in the PR description that this feature was shipped as part of rust 1.75 just so we know.

[Br1ght0ne]

(sorry if this was discussed previously, I could have missed it)

Took another look at this. Seems like some sort of type erasure needs to be done, as explained in erased-serde (by none other than dtolnay).

E.g., we would have ErasedSigner and impl Signer for dyn ErasedSigner, and use trait objects like these:

-unresolved_signatures: &[Box<dyn Signer + Send + Sync>],
+unresolved_signatures: &[Box<dyn ErasedSigner + Send + Sync>],

[segfault-magnet]

@MujkicA Any update?
@hal3e did you sort out the object safety issues with @MujkicA ?

[MujkicA]

We discussed using Rust's own async trait support and also seal traits like Transaction, TranscationBuilder etc.

Because of lack of support for dynamic dispatch, it was agreed that we stick to the async-trait crate for now.
There is no indication that we need the performance gain from not Boxing futures at trait boundaries (e.g. TransactionBuilder).