Streamline baker key registration/changes on the node

**Task description**

It is currently cumbersome both to register as a baker, as well as to change keys. It involves node restarts and copying files to a specific place. Concordium-client and other tools (e.g., desktop wallet) already handle baker keys when sending transactions so they can update the node as well.

The node already supports some baker management commands, such as start and stop baker. They are not really used much at this point, but they could be revised to support key management as well.

**Subtasks**
- [ ] Revise the node API/internals to support registering a baker. It is currently not possible to switch a node from a passive configuration to an active one, so registering a baker might be more work than we are willing to spend. But changing keys does not have this restriction.
- [ ] Use the new API to control the baker from concordium-client.

Read the existing node baker management commands, especially start and stop baker, then trace the node API/internals they use and the concordium-client key-management path. Clarify the scope for passive-to-active registration versus key changes; done means the supported flow works through the node API and concordium-client without restarts or manual file copying.

Written by the indexing model from the issue text.