♻️ Simplify SASL::{Name}Authenticator registration

In addition to selecting the constant based on the name, this also lazy
loads the authenticator definitions.  Most authenticators are never
used, so this avoids the cost of loading them all.

lib/net/imap/sasl/authenticators.rb

@@ -33,11 +33,11 @@ class Authenticators
     def initialize(use_defaults: false)
       @authenticators = {}
       if use_defaults
-        add_authenticator "Plain",      PlainAuthenticator
-        add_authenticator "XOAuth2",    XOAuth2Authenticator
-        add_authenticator "Login",      LoginAuthenticator     # deprecated
-        add_authenticator "Cram-MD5",   CramMD5Authenticator   # deprecated
-        add_authenticator "Digest-MD5", DigestMD5Authenticator # deprecated
+        add_authenticator "Plain"
+        add_authenticator "XOAuth2"
+        add_authenticator "Login"      # deprecated
+        add_authenticator "Cram-MD5"   # deprecated
+        add_authenticator "Digest-MD5" # deprecated
       end
     end
 
@@ -60,8 +60,16 @@ def names; @authenticators.keys end
     # When only a single argument is given, the authenticator class will be
     # lazily loaded from <tt>Net::IMAP::SASL::#{name}Authenticator</tt> (case is
     # preserved and non-alphanumeric characters are removed..
-    def add_authenticator(name, authenticator)
+    def add_authenticator(name, authenticator = nil)
       key = name.upcase.to_sym
+      authenticator ||= begin
+        class_name = "#{name.gsub(/[^a-zA-Z0-9]/, "")}Authenticator".to_sym
+        auth_class = nil
+        ->(*creds, **props, &block) {
+          auth_class ||= Net::IMAP::SASL.const_get(class_name)
+          auth_class.new(*creds, **props, &block)
+        }
+      end
       @authenticators[key] = authenticator
     end