`topology` currently contains the `requests` map, which is suitable for
node-specific requests such as "this node wants to join" or "this node
must be removed". But for requests for operations that affect the
cluster as a whole, a separate request type and field is more
appropriate. Introduce one.
The enum currently contains the option `new_cdc_generation` for requests
to create a new CDC generation in the cluster. We will implement the
whole procedure in later commits.
acfb6bf3ed