Design Signer Retirement mechanism

[jpraynaud]

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)