Design Signer Retirement mechanism

## Issue
In order to avoid having registered signer being retired too early, we could create a mechanism a la Cardano for signer retirement:
- The SPO adds option to specify the epoch when he wants to retire (> last epoch already registered + offset)
- The signer does not keep registering for new epochs with this option, but keeps signing for eligible epochs
- After the transition to the retirement epoch, the signer could stop and never start again (with the retirement epoch set)