In this commit, we modify the `GetPrice` method and interface to accept
the full request instead of _just_ the path. For backwards compat, we
leave the path in place, but also include the full serialized HTTP
request.
This commit adds a new pricer package which contains a Pricer interface
and two implementations of the interface. The Pricer interface can be
used to query the price of a certain path. The two implementations are
as follows: a DefaultPricer which returns the same price for all paths
of a service, and a GRPCPricer which queries a backend grpc server for
the price of a given path.
