Make the admin chain public.

[afck]

Ultimately the admin chain will likely have an admin application (e.g. staking) that has permissions to start new epochs and change the committee and resource policy. It will use only the public mode described in https://github.com/linera-io/linera-protocol/issues/1621, i.e. the validators are the only leaders. Round timeouts start once there is a message in the inbox.

• [ ] Application permissions for new epoch, committee, resource policy.
• [ ] Allow configuring such an admin chain in genesis (including the application).

Related to https://github.com/linera-io/linera-protocol/issues/263 and https://github.com/linera-io/linera-protocol/issues/464.