To make the permission system even more fine-grained, we want to allow
users to specify exact gRPC URIs in the macaroon permissions instead of
just broad entity/action groups.
For this we add the special entity "uri" which allows an URI specific
permission to be defined as "uri:/lnrpc.Lightning/GetInfo" for example
instead of the more coarse "info:read" which gives access to multiple
URIs.
