feat: Add public key to Block Header

[sergerad]

Context

Canonical block headers will require signatures of their contents from the validator.

The block headers need to contain the public key of the designated validator. Block headers need to provide a means to be signed by the designated validator.

Relates to 0xMiden/node#1316.

Changes

• Add BlockSigner trait.
• Replace proof commitment field with public key in BlockHeader.
• Add signature field to ProvenBlock.

[PhilippGackstatter]

Looks good!

I left a few comments and questions.

[sergerad]

@PhilippGackstatter the LocalEcdsaSigner did have the benefit of abstracting away nostd support for key generation:

    #[cfg(any(feature = "testing", test))]
    pub fn dummy() -> Self {
        use rand::SeedableRng;
        use rand_chacha::ChaCha20Rng;

        let mut rng = ChaCha20Rng::from_os_rng();
        LocalEcdsaSigner {
            secret_key: ecdsa::SecretKey::with_rng(&mut rng),
        }
    }

Thoughts on how to add a replacement for this now that we impl EcdsaSigner for SecretKey?

[PhilippGackstatter]

Thoughts on how to add a replacement for this now that we impl EcdsaSigner for SecretKey?

Maybe an extension trait feature-gated behind testing? But if that turns out annoying, we can also add back LocalEcdsaSigner, though maybe behind testing, too?

[PhilippGackstatter]

Looks good to me!

[bobbinth]

Looks good! Thank you! I left some comments inline.

Also, I'm assuming we don't want to introduce the concept of a SignedBlock yet, right?

[bobbinth]

Looks good! Thank you! I left some comments inline - pretty much all of them are nits (and majority are about renaming "public key" to "validator key").

[bobbinth]

Looks good! Thank you! A few more minor comments inline.

[mmagician]

LGTM, I left some nits mostly about the comments.
One thing I'd simplify though is whether we need random_signer.rs at all