Add a `Graceful Exit` mechanism for Nodes to exit DSL

[HenryQW]

A Node should be able to gracefully exit DSL by announcing its intention through an API endpoint offered by the broadcaster.

• This endpoint should verify the operator's identity via a signature
• After X epochs, the Node is removed from DSL and ceases to receive any requests and rewards (status = exited)
• Early exit will be slashed by enforcer
• /nta should always return the correct status