Ipfs submission API

For clients with short online period, a remote IPFS endpoint should be provided (see https://forum.duniter.org/t/soumission-via-endpoint-http-centralise-et-format-de-signature/12260).

A quick non-standard HTTP API was provided after #5 (closed), but a far better solution would be to use a restricted RPC API with only:

• dag/block put (not pin) / file add
• pubsub pub

This would allow to make data available to the network during the time necessary for pinning. This should be a separate ipfs node because an attacker could upload enough data to trigger garbage collection. A rate limiter could be setup to prevent this.