[feature]: allow authenticating the price oracle server with a macaroon

[ZZiigguurraatt]

Currently there appears to be no config option to specify a macaroon for authentication of the tapd price oracle client to the price oracle server. We are also lacking the ability to authenticate the price oracle server to the tapd instance (#1278), but hopefully that will be fixed soon. A price oracle server could have a deep view of the tapd node operator's portfolio and risk preference. A tapd node operator should be able to design a price oracle server with this intelligence and be able to authenticate its clients with a macaroon to allow operation of the price oracle on a non-private network. Also a price oracle may want to provide different results based on the macaroon that the client authenticates with.