♻️ SASL DIGEST-MD5: inherit, props, move, document

* ♻️  Inherit from Authenticator base class.
* 🚚 Move to sasl directory and SASL namespace.
* 🗑️ Deprecate original constant name.

Author: nevans

lib/net/imap/authenticators.rb

@@ -101,5 +101,5 @@ def authenticators
 
 require_relative "authenticators/login"
 require_relative "authenticators/cram_md5"
-require_relative "authenticators/digest_md5"
+require_relative "sasl/digest_md5_authenticator"
 require_relative "authenticators/xoauth2"