Client intended to work with an OPA server over HTTP(S).

Initial work is towards policy querying (using the Data API), not policy or document management within an OPA server.

Currently integration tests use (a fork of) testcontainers crate, which requires Docker or Podman to be running. The setup as it stands today is through using Unix domain sockets. If using some non-default socket location, DOCKER_SOCKET environment variable should be set, pointing to the control endpoint for Docker or Podman.