feat: implement managed control planes

Cluster template now has
```
managed:
  enable: true|false
```
configuration in the control planes model.
Creating managed control planes will be possible only if the Omni
account is configured with the provider to use for managed control
planes, otherwise resource validation won't allow creating the machine
set with such configuration.

Also drop `MachineRequestSet` allocation mode from the machine set,
remove source there, as it doesn't make much sense: for managed control
planes it should always use the machine request set name with the same
name as the machine set.

Config patches for managed mode are going to be added in the follow-up
PRs.

Fixes: siderolabs#705

Signed-off-by: Artem Chernyshev <artem.chernyshev@talos-systems.com>