Update monolith -api behaviour

[neilalexander]

This makes some changes to the monolith -api behaviour:

• The monolith internal listener is now on localhost:18008 by default when -api is enabled
• Adds -api-bind-address for overriding the above
• The setup package now prefers setting up the external listener over the internal one, which ultimately makes more sense because that's where the majority of useful HTTP routing goes

This probably requires a patch to Sytest to set -api-bind-address to something non-conflicting when we use -api.

[kegsay]

Why do we do this though?

[neilalexander]

So there are "external" and "internal" listeners. The external listeners are where /_matrix APIs go. The internal ones are where we put /api, /metrics. etc. Depending on whether we run in monolith mode or polylith mode, the behaviour is a bit different.

In monolith mode, we were previously defaulting to both listeners being the same, so /api, /metrics and other sensitive endpoints could end up on the public listener.

In polylith mode, whether a component starts both external+internal listeners or just the internal listener depends on whether it has public-facing APIs (like the client API, sync API, federation API, media API) or not.

This PR changes things around so that the internal listener now listens on localhost by default, ensuring that the internal routing now stays on the internal listener, unless explicitly overridden.

[kegsay]

What's going on here? You need some actual values here else NoInternalListener is the same as NoExternalListener - https://play.golang.org/p/bIAL8JumJtK

[neilalexander]

They're just designed to be human-readable rather than leaving "" littered about the place, e.g.

dendrite/cmd/dendrite-monolith-server/main.go

Lines 164 to 168 in 759bab6

	base.SetupAndServeHTTP(
	setup.NoInternalListener, // internal API
	httpsAddr, // external API
	certFile, keyFile, // TLS settings
	)

dendrite/cmd/dendrite-room-server/main.go

Lines 35 to 39 in 759bab6

	base.SetupAndServeHTTP(
	base.Cfg.RoomServer.InternalAPI.Listen,
	setup.NoExternalListener,
	nil, nil,
	)

[kegsay]

Maybe setup.NoListener instead?