Allow users to specify features for synthetic-monitoring-agent

[tylerauerbeck]

For the synthetic-monitoring-agent, a set of values can be provided to enable additional features of the agent. Currently it doesn't appear that this is possible as we can't pass in additional args or env vars to set this.

This is particularly impactful as if you haven't checked the docs, it's easy to miss that the adhoc feature is required in order to enable running the Test check from the Grafana UI.

[tylerauerbeck]

I'd be happy to contribute here. Curious if there's a preference to do this via args or env. I actually had some issues going the env route and was only able to resolve this by passing this in as a set of args by manually modifying the generated manifests.