Use a machine fingerprint for agent identification

https://redmine.teklia.com/issues/10050

• The Agent.public_key model field is replaced by an Agent.fingerprint CharField of 64 characters.

  • Only in the database migration, the field defaults to ''.
  • A UniqueConstraint should require unique fingerprints on all agents.

• Remove arkindex.ponos.serializer_fields.Base64Field and .PublicKeyField

  Those were only used for the public_key and derivation fields of CreateAgent in EE, which will be removed.