♻️ 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/authenticators/digest_md5.rb

@@ -65,14 +65,16 @@ module SASL
       autoload :PlainAuthenticator,       "#{sasl_dir}/plain_authenticator"
       autoload :XOAuth2Authenticator,     "#{sasl_dir}/xoauth2_authenticator"
 
+      autoload :DigestMD5Authenticator,   "#{sasl_dir}/digest_md5_authenticator"
+
       # Authenticators are all lazy loaded
       def self.authenticators
         @authenticators ||= SASL::Authenticators.new.tap do |registry|
           registry.add_authenticator "Plain"
           registry.add_authenticator "XOAuth2"
           registry.add_authenticator "Login",      LoginAuthenticator     # deprecated
           registry.add_authenticator "Cram-MD5",   CramMD5Authenticator   # deprecated
-          registry.add_authenticator "Digest-MD5", DigestMD5Authenticator # deprecated
+          registry.add_authenticator "Digest-MD5" # deprecated
         end
       end
 
@@ -109,4 +111,3 @@ def initial_response?(mechanism)
 
 require_relative "authenticators/login"
 require_relative "authenticators/cram_md5"
-require_relative "authenticators/digest_md5"