feat(consensus): signer field in Signed tx.

[yash-atreya]

Motivation

Ref #481

Solution

Added signer: OnceLock<Address> in Signed

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes

[prestwich]

proposed api

• delete recover_signer as standard API
• add new_with_unchecked_signer for instantiation when signer is known (e.g. when we just signed)
• modify signer as follows:

/// get the signer if known or recoverable
fn signer(&self) -> Option<Address> {
    #[cfg(not(feature = "k256"))]
    self.signer.get().copied()

    #[cfg(feature = "k256")]
    *self.signer.get_or_init(|| { /* recovery logic here */ })
}

[prestwich]

also ok if implemented as

#[cfg(not(feature = "k256"))]
fn signer(&self) -> Option<Address> { ... }

#[cfg(feature = "k256")]
fn signer(&self) -> Option<Address> { ... }

whatever feels cleaner

[prestwich]

I did a little bit of work here, and because we're attempting to be generic over Sig, this is harder than it seems without a Recoverable trait for the signature type. which i'm not sure we want to introduce yet. Going to close this as needing more design work

another problem specialization would solve 🤔