Conan Skeleton API

Proposal:

Implement skeleton Conan server API so that we can start adding endpoints. This can be very basic but should cover all the base requirements we need. I suggest we start with GET /v1/ping endpoint that will return 401 Unauthorized when the client can access it and 404 Not Found otherwise. It should be behind a feature flag and available only for the correct plans/tiers.

Once we have this in a subsequent issue we will add authentication and change it to return 200 OK and empty list of server capabilities. Then we can start building the endpoints we need on top of that.

Here is the implementation of GET /v1/ping from the conan_server itself - https://github.com/conan-io/conan/blob/develop/conans/server/rest/controller/common/ping.py.

We can also use the Maven packages API as example - https://gitlab.com/gitlab-org/gitlab-ee/blob/master/ee/lib/api/maven_packages.rb.