Append endpoint DAG surgery

[icidasset]

This PR implements most of the functionality needed for the HTTP endpoint that appends a file to the DAG of an app.

Based on the append-endpoint branch.

• Adds various remote IPFS functions for working with the MFS
• Adds a function to add data to the DAG of an app, and another that calls it, and sets the new CID in the database + DNSLink
• Adds some additional info the server readme

This doesn't check for the correct resources in the provided UCAN, that'll be a separate PR.

PUT "http://runfission.test:1337/v2/api/append/app-name/file.name"

Content-Type: application/octet-stream
Authorization: Bearer UCAN

204 No Content