feat: support remote attachments

[dmccartney]

This adds support for remote attachments to the RN SDK.

Goals: do heavy lifting in native code and minimize data sent across JS/native bridge (i.e. use file references).

Note: this is the binding for remoteAttachment leftover from #81 and uses a similar approach to abstracting codec usage.

Sending:

• the app helps the sender select a fileUri to send
• the app asks XMTP to encrypt the file: client.encryptAttachment({ fileUri, ... })
• the app uploads the encryptedLocalFileUri to web storage to get a url
• the app asks XMTP to send the message: client.send({ remoteAttachment: { url, ... }})

Receiving:

• the app lists messages from XMTP having a message.content.remoteAttachment
• when the receiver taps the attachment, the app fetches the url to an encryptedLocalFileUri
• the app asks XMTP to decrypt the local file: client.decryptAttachment({ encryptedLocalFileUri, ...})
• the app presents the decrypted fileUri to the user based on the mimeType

This is the high-level TODO list:

• sending
  • JS interface + demo sending UI
  • iOS sending
  • Android sending
• receiving
  • JS interface + demo receiving UI
  • iOS receiving
  • Android receiving
• misc
  • cleanup the ConversationScreen code in the demo app
  • find a less hacky dev rigging for upload-service testing

[github-actions]

🎉 This PR is included in version 1.10.0 🎉

The release is available on:

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

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