Merge api3dao/chains into this repo

[bbenligiray]

api3dao/chains will be merged to this repo and @api3/contracts will include @api3/chains features, but @api3/chains will continue being maintained until all users migrate. One idea is to treat @api3/chains feature as beta versions until that happens by preceding them with _ (which would mean that we can do breaking updates to them without a major release).