Rotate cryptographic keys for ActivityPub Server 2 Server communication

[HelgeKrueger]

Implement automatic rotation of the public key pair used for ActivityPub Server 2 Server communication after say a month:

• Investigate best practices on the period to update a keypair
• Ensure updating the key pair doesn't create problems when communicating with other FediVerse services
• Investigate if there is a way to inform other services about an updated keypair. Otherwise consider writing a FEP

Automatic rotation will probably implemented as part of Moo-Auth-1 as the key is part of the header there, rotation should be unproblematic.