Modify schnorr and ecdsa account contracts to store the public key

[PhilWindle]

Now that we have account contracts working with Schnorr and ECDSA signatures, it would be great to update these so we don't have to pass in the public key for signature verification. Instead, we should provide an initial public key at contract creation. This will need to be nullified and re-created with each read.

Once this is achieved, the next step will be to add a method by which the public key can be rotated.

[spalladino]

Key rotation is blocked until we have transient commitments. For now, we can implement this as a private constant note, so we don't need to nullify it on every read. Consider using ImmutableSingleton in Noir to implement it.