We should start each test with clean configs to prevent them from
interfering with eachother.

A follow up commit adds kafka/types.h as a transitive dependency to the
client_quota_translator_test.cc which causes the client_id constant in
the test file to clash with the client_id type alias from types.h.

This fixes a bug in the quota values being returned for group-based
quota keys where we would incorrectly fall back to the default values
for group-based keys when the correct quota is really "not applicable"
(i.e. `std::nullopt`).

This is not an observable bug because these quota values are never
really accessed. If the request had a group-based applicable quota, it
would both have a group-based key and its limit. It's not possible for a
request to have a group-based key and a default quota value.

We fix this bug now because we are about to integrate with the quota
store, which makes this bug more apparent.

This wires the sharded quota_store into the client_quota_translator.
There's no behaviour change yet (the quota store is not accessed yet),
so this is merely a refactor.

This refactors the quota lookup methods to be generic across the
different request types in a way that allows us to hook in quotas from
the quota store in follow up commits.

Refactor only, no behaviour change.