Summary:
This enables running server control as RPC to node agent. There are some features + refactoring changes. The feature is gated by a global runtime feature flag.

1. Implement java client + node agent server method for server control.
2. The file shell_task.go is split into the actual command creator and the task runner. The reason is that we will modules running command and the dependency is cleaner (no circular).
3. Node agent dead detector reduced to about 20mins.
4. proto file is split into 2 - 1. the actual core service 2. YB server related ones.

Test Plan:
1. UT added for basic command generator.
2. Created a universe with this change, scaled out, stopped/started a tserver (after enabling this feature).

ServerControl RPC is very fast. Stop tserver (no configure call) just ran very quickly.

Reviewers: svarshney, yshchetinin, anijhawan, nbhatia

Reviewed By: yshchetinin

Subscribers: yugaware

Differential Revision: https://phorge.dev.yugabyte.com/D43202