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>

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