Use SENDER authentication instead of signatures

[sbihel]

Reasons:

• makes the smart contract and the interactions with the entrypoints simpler;
• the generic rotation events do not provide any benefit, we can simply retrieve the history of calls (and it also makes the smart contract simpler with fewer PACKs); and
• the difference between SENDER and SOURCE still allow for a third party to execute the operations on behalf of the owner.

TODO:

• [ ] Update the smart contract
• [ ] Update the specs

CC @michaeljklein