Implement `cmd/createdkg`

### Description
Implement `pluto create dkg` following the reference files:

- `cmd/createdkg.go`
- `cmd/createenr_internal_test.go`

### Benefit

Gets us closer to full DKG compatibility, without requiring any actual `dkg/` modules to be implemented.

### Acceptance Criteria
All unit tests from the reference Go implementation must be ported and must pass.
