Support externally managed secret keys

[kasperisager]

This pull request introduces support for externally managed secret keys to allow a relaying DHT node to act on behalf of another relayed node. A summary of the changes:

• DHT.createServer() and DHT.connect() now accept two new options, createHandshake and createSecretStream, to allow callers to perform Noise handshaking and stream setup themselves. To ensure that the relaying DHT node can still hole punch on behalf of the relayed node, handshake.final() is now required to return a holepunchSecret key, but may leave out the Noise keys.

• server.listen() now accepts two new options, signAnnounce and signUnannounce, to allow callers to perform signing themselves.

• DHT.lookupAndUnannounce(), DHT.unannounce(), and DHT.announce() now accept a new option, signUnannounce, to allow callers to perform signing themselves. DHT.announce() also accepts a signAnnounce option and DHT.mutablePut() accepts a signMutable option.

[kasperisager]

~@davidmarkclements The tests are failing on Windows as of Brittle 2.0.2. Do you know what might be causing this?~

This got sorted in 2.0.3 👌

[kasperisager]

@mafintosh This is ready for another look.

[kasperisager]

I broke the tests 💥