metadata is not available to callbacks for EventSub events received via WebSocket

[alinsavix]

As per discussion at https://discord.com/channels/325552783787032576/785942102386540614/1328016079670480946 , the data provided to callbacks handling incoming EventSub notifications via WebSocket does not provide any way to access much of the data from the metadata key of the incoming json message. In particular, the metadata.message_timestamp and metadata.message_id fields are both useful, and not available.

Reference for incoming notification messages: https://dev.twitch.tv/docs/eventsub/websocket-reference/#notification-message

As noted in the discord thread, the similarly named fields under the subscription key are not the same thing -- in particular, the id under that key is the id for the subscription, and the created_at under that key is when the subscription was created.

Most of the other metadata fields (message_type, subscription_type, and subscription_version) are already covered, either because they never change (message_type), or because they're available in the subscription field (subscription_type and subscription_version).

Thanks!