feat: delegate signers of batches

[significance]

it would be useful for owners of a batch to be able to delegate signing of stamps to other key pairs.

in this way, someone could create a batch, and then allow someone else to be responsible for filling the storage slots, without divulging their own private key.

[0xCardinalError]

Probably we are aiming here for something like here also seems duplicate then and would close this in favour of that issue
#166