Move the monitor controller into pkg/enterprise

[caseydavenport]

Description

First of the controller relocations for CORE-13484. The monitoring controller is Enterprise-only, so it moves under the Enterprise package along with the registration seam the rest of the moves need.

• A variant contributes its own controllers through a list on the controller options. The core manager setup iterates that list rather than naming each type, so it no longer knows the Enterprise controllers exist.
• Registration is what gates a contributed controller, so the moved controller drops its own variant check.
• The Enterprise tree mirrors the core tree it came from, so a controller lands in pkg/enterprise/controller and its render code will land in pkg/enterprise/render. Flat would collide with the extension packages already sharing that directory.
• The render package stays where it is. Several other Enterprise controllers still depend on it, and it moves once they do.

The rest of the Enterprise-only controllers follow in separate PRs, one subsystem at a time.

Related: CORE-13484

Release Note

None