[AIO] Registering a connection needs to be split out of connect()

To implement CAP negotiations (for SASL) before registering a connection I had to split connection registration out of `AioConnection::connect()`: https://github.com/hifi/heisenbridge/blob/master/heisenbridge/irc.py#L120

My implementation is currently ugly but the requirement remains that registration needs to be split out either optionally (arg for connect to disable?) or by changing the API to require calling `register()` after `connect()`.

I can PR an accepted solution.

Start with AioConnection::connect() and compare the registration flow with the referenced heisenbridge/irc.py example around line 120. Determine whether registration should be optional during connect() or exposed as a separate register() call, then ensure CAP/SASL negotiation can occur before registration and document the accepted API behavior.

Written by the indexing model from the issue text.