Allow submitting data through GraphQL mutation

Although Datapods were made with p2p in mind, it is not always relevant or possible to embark an ipfs node in an app. For example in Dart or very light client.

Thus, user should be able to submit data through a trusted relay, like a GraphQL endpoint, which will then send the message over pubsub for other Datapods to be informed.

This also means that the default message format which consist in signing a CID might not be the easiest option because it requires a multiformats implementation.

An alternative signing format more similar to Cesium+ could be added, knowing that this will require custom signature verification on server side.