🚧 Add ExtensionData for unhandled extensions

Although this is currently unused, it should eventually be used for
`StatusData`, `BodyStructure`, `ESEARCH`, `MailboxList`, etc.

Author: nevans

lib/net/imap/response_data.rb

@@ -72,7 +72,7 @@ class IgnoredResponse < UntaggedResponse
     # unknown extensions to response types without a well-defined extension
     # grammar.
     #
-    # See also: UnparsedNumericResponseData
+    # See also: UnparsedNumericResponseData, ExtensionData, IgnoredResponse
     class UnparsedData < Struct.new(:unparsed_data)
       ##
       # method: unparsed_data
@@ -88,7 +88,7 @@ class UnparsedData < Struct.new(:unparsed_data)
     # Net::IMAP::UnparsedNumericResponseData represents data for unhandled
     # response types with a numeric prefix.  See the documentation for #number.
     #
-    # See also: UnparsedData
+    # See also: UnparsedData, ExtensionData, IgnoredResponse
     class UnparsedNumericResponseData < Struct.new(:number, :unparsed_data)
       ##
       # method: number
@@ -107,6 +107,23 @@ class UnparsedNumericResponseData < Struct.new(:number, :unparsed_data)
       # The unparsed data, not including #number or UntaggedResponse#name.
     end
 
+    # **Note:** This represents an intentionally _unstable_ API.  Where
+    # instances of this class are returned, future releases may return a
+    # different (incompatible) object <em>without deprecation or warning</em>.
+    #
+    # Net::IMAP::ExtensionData represents data that is parsable according to the
+    # forward-compatible extension syntax in RFC3501, RFC4466, or RFC9051, but
+    # isn't directly known or understood by Net::IMAP yet.
+    #
+    # See also: UnparsedData, UnparsedNumericResponseData, IgnoredResponse
+    class ExtensionData < Struct.new(:data)
+      ##
+      # method: data
+      # :call-seq: data -> string
+      #
+      # The parsed extension data.
+    end
+
     # Net::IMAP::TaggedResponse represents tagged responses.
     #
     # The server completion result response indicates the success or