feat: A new field custom_emoji_metadata is added to message `.googl…

…e.chat.v1.Annotation`

feat: A new message `CustomEmojiMetadata` is added
feat: A new value `CUSTOM_EMOJI` is added to enum `AnnotationType`
docs: A comment for method `CreateReaction` in service `ChatService` is changed
docs: A comment for method `DeleteReaction` in service `ChatService` is changed
docs: A comment for field `custom_emoji` in message `.google.chat.v1.Emoji` is changed

PiperOrigin-RevId: 720564143

Source-Link: googleapis/googleapis@60bab91

Source-Link: googleapis/googleapis-gen@f7beff2
Copy-Tag: eyJwIjoiYXBpcy9Hb29nbGUuQXBwcy5DaGF0LlYxLy5Pd2xCb3QueWFtbCIsImgiOiJmN2JlZmYyOTQ3ODc1MmQ4NjY5MWQ4MTU0ODE1YTg1MzUzYTY3MzMzIn0=