Consider using a canonical DKIMRegistry address for ZKEmail contracts

[ernestognw]

In previous ZKEmail work, a canonical DKIMRegistry address that is the same across all chains might be the best recommendation for users, so we should consider providing such address as a default.

          > Is this a canonical address or one particular to the DKIM public key?

actually, we do envision this as a canonical address per chain, as this is the dkim registry that get's updated by our infrastructure. Users can also override the dkim keys on the canonical registry if they want

Ofc users can still deploy their own but they would have to update it themselves.

This is not clear from our docs so we will update them!

Originally posted by @JohnGuilding in #103 (comment)

In a similar fashion, we do offer a hardcoded entrypoint in ERC4337Utils