Adds support for listening on and connecting to I2P and Onion services securely (#3293)

This PR adds 2 `dendrite-demo` main's, each designed expressly to serve
a Hidden Service/Overlay network.

The first, `dendrite-demo-i2p` add self-configuration for use of
dendrite as an I2P hidden service(eepsite) and to connect to I2P
services(federate) as an I2P client. It further disables the `dendrite`
server from communicating with non-anonymous servers by
federation(because I2P does not canonically have the ability to exit, we
rely on donors for exit traffic), and enables the use of self-signed TLS
certificates([because I2P services are self-authenticating but TLS is
still required for other aspects of the system to work
reliably](https://tor.stackexchange.com/questions/13887/registering-onion-with-certificate-authority)).
This demo turns the system into an "pseudonymous" homeserver which
people can connect to using an I2P-enabled Matrix client(I like `cinny`
and it's what I tested with).

The second, `dendrite-demo-tor` adds self-configuration for the use of
dendrite as an Onion service and to connect to other onion services and
non-anonymous web sites using Tor to obfuscate it's physical location
and providing, optionally, pseudonymity. It also enables the use of
self-signed TLS certificates, for the same reason as with I2P, because
onion services aren't typically eligible for TLS certificates. It has
also been tested with `cinny`.

These services are both pseudonymous like myself, not anonymous. I will
be meeting members of the element team at the CCC assembly shortly to
discuss contributing under my pseudonym.

As none of the other `dendrite-demo` have unit tests I did not add them
to these checkins.

* [*] I have added Go unit tests or [Complement integration
tests](https://github.com/matrix-org/complement) for this PR _or_ I have
justified why this PR doesn't need tests

---------

Co-authored-by: eyedeekay <idk@mulder>
Co-authored-by: Till Faelligen <2353100+S7evinK@users.noreply.github.com>

contrib/dendrite-demo-tor/main_test.go

@@ -0,0 +1,50 @@
+package main
+
+import (
+	"os"
+	"os/signal"
+	"strings"
+	"syscall"
+	"testing"
+)
+
+// This is an instrumented main, used when running integration tests (sytest) with code coverage.
+// Compile: go test -c -race -cover -covermode=atomic -o monolith.debug -coverpkg "github.com/matrix-org/..." ./cmd/dendrite
+// Run the monolith: ./monolith.debug -test.coverprofile=/somewhere/to/dump/integrationcover.out DEVEL --config dendrite.yaml
+// Generate HTML with coverage: go tool cover -html=/somewhere/where/there/is/integrationcover.out -o cover.html
+// Source: https://dzone.com/articles/measuring-integration-test-coverage-rate-in-pouchc
+func TestMain(t *testing.T) {
+	var args []string
+
+	for _, arg := range os.Args {
+		switch {
+		case strings.HasPrefix(arg, "DEVEL"):
+		case strings.HasPrefix(arg, "-test"):
+		default:
+			args = append(args, arg)
+		}
+	}
+
+	// only run the tests if there are args to be passed
+	if len(args) <= 1 {
+		return
+	}
+	t.Log(args)
+
+	waitCh := make(chan int, 1)
+	os.Args = args
+	go func() {
+		main()
+		close(waitCh)
+	}()
+
+	signalCh := make(chan os.Signal, 1)
+	signal.Notify(signalCh, syscall.SIGINT, syscall.SIGQUIT, syscall.SIGTERM, syscall.SIGHUP)
+
+	select {
+	case <-signalCh:
+		return
+	case <-waitCh:
+		return
+	}
+}