The incorrect assumption that slugs were unique per-agent was made when
the subagent API was implemented. Whilst this PR doesn't completely
enforce that, we instead compute a stable hash to prefix the slug that
should provide a reasonable level of probability that the slug will be
unique.
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.
