Completely missed this in the original PR for adding support for
creating sub agents. This now allows specifying a list of display apps
to be added to the agent.
Closes https://github.com/coder/internal/issues/619
Implement the `coderd` side of the AgentAPI for the upcoming
dev-container agents work.
`agent/agenttest/client.go` is left unimplemented for a future PR
working to implement the agent side of this feature.