Disable mappings endpoint related infrastructure when endpoint isn't available

[wilkinsona]

From some analysis by @aclement, when the mappings endpoint is unavailable we still auto-configure some beans that, generally speaking, are only used by the endpoint. On the servlet side we'll typically define three beans that aren't needed and on the reactive side we'll define one.

While it's not beyond the realms of possibility that someone is using the defined beans without using the endpoint, I think it's unlikely. We should consider making all the infrastructure conditional on the mappings endpoint being available so that users just pay (in startup time and memory footprint) for what they're using.

[wilkinsona]

There may be other areas in Actuator where we could make a similar change. Flagging for team attention to see how far, if at all, we want to take this.