feat: new multi remote attachment content type

[cameronvoell]

See https://github.com/xmtp/XIPs/blob/main/XIPs/xip-50-multiple-remote-attachments-content-type.md

This XIP proposes a new MultiRemoteAttachment content type inspired by the existing RemoteAttachment content type, that instead allows for multiple remote attachments to be sent in a single message.
Where the RemoteAttachment content is a URL pointing to an encrypted protobuf EncodedContent, the MultiRemoteAttachment content is a protobuf struct containing an array of remote attachment structs, each specifying a URL, as well as a contentDigest, contentLength, nonce, scheme, and filename.
The idea being that the multiple remote attachment encoded content parameters will specify a secret key and salt for encrypting/decrypting all attachments, but each attachment will have its own nonce, and contentDigest for validating the integrity of the individual attachments.

[github-actions]

🎉 This PR is included in version 3.74.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀