feat: run embedded discovery service in Omni

[utkuozdemir]

Depends on https://github.com/siderolabs/discovery-service/pull/56

Run a discovery service instance inside Omni, which can be disabled but is enabled by default. When enabled, it will listen only on the SideroLink interface on port 9093. When enabled, all new clusters will use the embedded discovery service instead of the public one.

Existing clusters will remain unaffected by this change.

Closes siderolabs/omni#20.