Support DIDComm Messaging attachments

[awoie]

TBD: needs more refinement

Extend the current DIDCommMessage and Message interfaces to support DIDCommV2 attachments as defined here: https://identity.foundation/didcomm-messaging/spec/#attachments

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[codynhat]

I currently have a use case where attachments is needed. Currently, the DIDCommMessageHandler drops the attachments header before forwarding any unpacked messages. I am interested in taking on this work and opening a PR.

A little more context about my use case. I am developing a set of DIDComm protocols that will use the IPLD DAG-JSON codec as a way to specify links to other content-addressable content. This content could be external to the DIDComm message, or can be an attachment in the message. I would like to be able to handle these attachments using Veramo, and also develop a new message handler that does the following:

1. Decodes a DIDComm body from the DIDCommMessageHandler as DAG-JSON, essentially turning links into CID objects
2. Packages the body and all attachments into an in-memory CAR as the raw field on the Message

Message handlers specific to these DIDComm protocols that use DAG-JSON can then use IPLD tooling internally to traverse the message and its links to attachments using the CAR.

Here is an example of what a message using DAG-JSON and some attachments would look like:

{
    "id": "...",
    "type": "...",
    "body": {
        "someLinkedContent": { "/": "<CID of offer>" },
    },
    "attachments": [
        {
          "data": {
            "hash": "<multi hash of content>"
            "json": {
              ... some attached JSON content
            }
          }
        }
    ]
}