[routing-manager] new APIs to get state and new CLI commands (#8784)

This commit adds new public OT APIs in `border_routing.h` to get the
current state of `RoutingManager` and to get the current favored
on-link prefix.

This commit also updates CLI `br` commands:
- It moves the CLI `br` implementation into own `cli_br` module.
- It adds `br state` to get the current state
- It changes `br omrprefix`, `br onlinkprefix`, and `br nat64prefix`
  to output both local and favored prefixes when no additional arg
  is provided and allow the use of `local` or `favored` extra arg to
  specify the type.
- It updates the documentation of `br` related commands in the source
  code and also adding `cli/README_BR.md`.

tools/harness-thci/OpenThread.py

@@ -3214,7 +3214,7 @@ class OpenThreadTHCI(object):
     @watched
     def isBorderRoutingEnabled(self):
         try:
-            self.__executeCommand('br omrprefix')
+            self.__executeCommand('br omrprefix local')
             return True
         except CommandError:
             return False