Add support for message snapshots (forwarding)

[QuantumToasted]

Description

https://discord.com/developers/docs/resources/message#message-reference-content-attribution-forwards

This PR adds support for receiving and sending message snapshots (forwarded messages).

Notable changes:

• MessageReferenceType enum added, with values Reply (0) and Forward (1)
• IMessageReference now contains a MessageReferenceType Type property
• Added IMessageSnapshot interface and Transient implementation - this type is very similar to IMessage but lacks an author property. I'm unsure if it would be worth it to abstract common data between the two types further, so I elected not to.
• MessageJsonModel is used as the model for snapshots - this introduces a "breaking" change where Author is optional (snapshots lack an author). Several models were adjusted where Author is known to have a value to simply use .Value now.

Received forwarded messages can be found via IUserMessage#MessageSnapshots, and can be sent via a LocalMessageReference with Type: Forward (not supported in app command responses)

Checklist

• I discussed this PR with the maintainer(s) prior to opening it.
• I read the contributing guidelines.
• I tested the changes in this PR.