feat: add helper to eip155 variant

[mattsse]

sometimes we need special checks for 155 or need to know if it is 155

adds two helpers for this

ref https://github.com/bluealloy/revm/pull/1824/files

[DaniPopes]

This is too specific, doesn't make sense to have as a method, is_ and as_ are enough

[mattsse]

hmm this would make things like:

            // Check y_parity
            if let Parity::Eip155(parity) = auth.signature().v() {
                if parity > u8::MAX as u64 {
                    return Err(InvalidAuthorization::InvalidYParity);
                }
            }
            Ok(())

slightly easier imo

[DaniPopes]

the same is achievable with as_eip().is_some_and(), again this is too specific

[mattsse]

still think this is nice to have and doesn't hurt really, but closing because can be solved with some extra calls as well