[cmd/opampsupervisor] Forward Custom Messages to/from agent

[BinaryFissionGames]

Component(s)

cmd/opampsupervisor

Is your feature request related to a problem? Please describe.

Custom messages are supported in the opamp extension, but the supervisor does not forward custom messages, so they cannot be used with the supervisor.

Describe the solution you'd like

Custom messages should be forwarded to/from the agent so that the agent can handle custom messages and send custom messages back to the opamp server.

Describe alternatives you've considered

No response

Additional context

No response

[github-actions]

Pinging code owners:

• cmd/opampsupervisor: @evan-bradley @atoulme @tigrannajaryan

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