This is a preparatory PR to add additional APIs (Pinning Service API) easily
to cluster.
Instead of copy-pasting most of what the REST API does, I have refactored so
that the whole configuration, routing and request-handling utilities can be
re-used.
The worst part has been to divide the test between tests that test core
(common.API) functionality and tests that test specific REST API endpoint
functionality. I could not get away without an additional common/test package
to provide functions that are used from both places. This is a side effect of
testing both http and libp2p endpoints for every request etc.
