tools/scylla_types: single source of truth for actions
tools/scylla_types: single source of truth for actions

Currently the available actions are documented in several different
places:
* code implementing them
* description
* documentation for --action
* error message that validates value for --action

This is guaranteed to result in incorrect, possibly self-contradicting
documentation. Resolve by generating all documentation from the handler
registry, which now also contains the description of the action.
Also have a separate flag for each action, instead of --action=$ACTION.

avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla