Summary:
## This diff: Add debugger mode infrastructure
We want to enable infrastructure allowing adding modes in the future without branching client and plugin code
This infra allows for the client to send a message informing flipper about the supported modes (following diffs)
Currently broadcasting mode change event over to the client
Reviewed By: lblasa
Differential Revision: D49385860
fbshipit-source-id: 2db6f65064d1ff7e1f57b2da93c3ed195400fb7f
1c3720fd5c