Make proxy routing logic reusable

[rosstimothy]

Routing logic existed within an unexported handler of ssh subsystem
requests, which prevented it from being reused by other components
within the proxy, like the webapi server. This causes significant
latency issues for web sessions because the web apiserver is required
to dial the proxy ssh server to determine how to route to the host.
Since the web apiserver and the proxy ssh server exist in the same
process this is an entirely unnecesarry step that could be avoided
if the routing and ability to established connections were shared
throughout the proxy.

A new proxy.Router object is introduced which contains all the
logic that used to exist in regular.proxySubsys for determining
how to connect to servers and clusters. All routing within the
regular.proxySubsys now leverages the proxy.Router to dial
the target.

This is step 1 in addressing #15167. Now that the proxy.Router
exists web.APIServer will be able to make use of it to avoid
dialing the same process to establish connections.

Note to reviewers:

This PR will be easier to review commit by commit as the first commit refactors lib/proxy so that all proxy peering code resides in lib/proxy/peer, which adds a bit of noise, but ultimately makes lib/proxy a bit tidier. The second commit adds proxy.Router, replaces the existing logic within regular.proxySubsys to use it, and updates all initializations of regular.Server which were supplying the SetProxyMode option to pass in a proxy.Router.

[rosstimothy]

friendly ping @alistanis @fspmarshall

[zmb3]

I thought you didn't like doing this in init?

(I actually prefer it this way, FWIW)

[rosstimothy]

Lesser or two evils. Doing it on every proxy subsystem request seemed a bit excessive.

teleport/lib/srv/regular/proxy.go

Lines 192 to 196 in 9679315

	func newProxySubsys(ctx *srv.ServerContext, srv *Server, req proxySubsysRequest) (*proxySubsys, error) {
	err := metrics.RegisterPrometheusCollectors(prometheusCollectors...)
	if err != nil {
	return nil, trace.Wrap(err)
	}

[github-actions]

@rosstimothy See the table below for backport results.

Branch	Result
branch/v10	Failed
branch/v11	Failed
branch/v9	Failed