With the new condition and checker in place, we can give the end user
the ability to add such a custom caveat to a baked macaroon.
There won't be an RPC counterpart for this operation since all first party
caveats currently are only added on the client side.
Due to a misunderstanding about how the entities/actions are encoded
inside the macaroon, only the first action was printed per entity.
Even though we add them as separate pairs in the macaroon service (for
example "offchain:read" and "offchain:write"), they are grouped in the
serialized macaroon ("offchain:read,write").
