Implement transaction comments

Duniter v1 included on-chain transaction comments which made them benefit from consensus.

In Duniter v2, we first thought about having a fully offchain implementation of transaction comments, but then came to the conclusion that it was better to keep the consensus on this data: https://forum.duniter.org/t/options-pour-gerer-les-commentaires-de-transaction-cote-serveur/12241/6

See https://forum.duniter.org/tag/commentaires for more info.

However, the way we would use these transaction comments can vary. For example:

• publishing a CID of the comment would allow to ask people to forget what this CID corresponds to
• the comment can be encrypted (but that's more a client-side question)