And use that from the existing accountFromMacaroon helper (which will
then test the new helper by proxy).
We add this helper so that we can use it later on from the sessions
package where we want to extract an account ID from a caveat (we wont
have a full macaroon available).
Update the accounts `Store` and `Service` interfaces take a context.
This is in preparation for when the backend DB of the accounts service
is a SQL store which will have methods that take a context.
In preperation for the trace logging we want to implement in an upcoming
commit (where logs for requests & responsescan be linked via their
request ID), we start adding the request ID to an intercepted account
request/response in this commit.
This commit introduces the account interceptor service which makes sure
that each request that contains an account macaroon is intercepted and
passed through the checkers component to track each RPC call's effects
on the account.
