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

* 🚚 Move to sasl directory and SASL namespace.
* 🗑️ Deprecate original constant name.
* ♻️  Inherit from Authenticator base class.
* ♻️  Refactor to the style used in the new ScramAuthenticator.

Yes, DIGEST-MD5 is deprecated!  But that also means that it is lower
risk as a test-bed for refactoring a more complicated challenge/response
SASL mechanism, and for developing a reasonable SASL::Authenticator base
class.  I improved it in several ways:

* 🔒 Use SecureRandom for cnonce (not Time.now + insecure PRNG!)
* ✨ Default qop=auth (as in RFC)
* ✨ User can configure realm, host, service_name, service.
  * This allows a correct "digest-uri" for non-IMAP clients.
* ✨ Enforce requirements for sparam keys (required and no-multiples).

However... it's still deprecated, so maybe don't use unless you need to!

Author: nevans

lib/net/imap.rb

@@ -996,7 +996,7 @@ def starttls(options = {}, verify = true)
     #   *Deprecated:*  <em>Obsolete mechanisms are available for backwards
     #   compatibility.</em>
     #
-    #   For +DIGEST-MD5+ see DigestMD5Authenticator.
+    #   For +DIGEST-MD5+ see SASL::DigestMD5Authenticator.
     #
     #   For +LOGIN+, see LoginAuthenticator.
     #

lib/net/imap/authenticators.rb

@@ -73,4 +73,4 @@ def authenticators
 # deprecated
 require_relative "authenticators/login"
 require_relative "authenticators/cram_md5"
-require_relative "authenticators/digest_md5"
+require_relative "sasl/digest_md5_authenticator"

lib/net/imap/authenticators/digest_md5.rb

@@ -31,7 +31,7 @@ module SASL
       #   *Deprecated:*  <em>Obsolete mechanisms are available for backwards
       #   compatibility.</em>
       #
-      #   For +DIGEST-MD5+ see DigestMD5Authenticator.
+      #   For +DIGEST-MD5+ see SASL::DigestMD5Authenticator.
       #
       #   For +LOGIN+, see LoginAuthenticator.
       #