🔒 Add SASL SCRAM-SHA-* mechanisms

[nevans]

Loosely based an initial implementation by @singpolyma at
nevans/net-sasl#5

• Depends on 🔒 Verify SASL authentication has completed #179
• Fixes SCRAM-SHA-1(-PLUS) + SCRAM-SHA-256(-PLUS) + SCRAM-SHA-512(-PLUS) + SCRAM-SHA3-512(-PLUS) supports #54

[nevans]

I've delayed finishing this PR for ~a year now? Unfortunately, I haven't had much cause to use this myself (OAUTHBEARER support was more immediately useful for my needs).

As far as the authenticator API (expanded to include #initial_response? and #done?) is concerned, it's basically done... although I would really appreciate another pair of eyes on the implementation!

However, Net::IMAP#authenticate still needs to be updated so it ensures the authenticator is #done?. As it is now, if the server returns OK before sending its final message, then we can't satisfy the following requirement from the RFC:

The client then authenticates the server by computing the
ServerSignature and comparing it to the value sent by the server. If
the two are different, the client MUST consider the authentication
exchange to be unsuccessful, and it might have to drop the
connection.

[nevans]

@singpolyma, @Neustradamus FYI: This has been merged and I'm going to make a new release containing it this week (maybe today or tomorrow?). If you have any time to look over what I've done and maybe even test it a little bit, I'd be grateful.

[Neustradamus]

@nevans: Excellent, thanks!

Have you plan a net-sasl build with?

• https://github.com/nevans/net-sasl

Have you looked for -PLUS variants part?

@singpolyma: Can you look?