Implement endpoint to perform request to services running in builds

As a part of https://gitlab.com/gitlab-org/gitlab-ee/issues/5276 we need a new endpoint (possibly in the EE::JobsController) that will generate the proper metadata that would allow performing requests to services running in the build in a specific port.

In order to make this work we would need also gitlab-runner#3948 (closed) and gitlab-workhorse#205 (closed). Once the Web IDE performs a request to a specific endpoint in Workhorse, it will receive a request that will be modified with the proper metadata to pass the Workhorse validations and finally make the request get to the runner.

This metadata will be:

• the url that we want to send to the runner. It will have the format /session/uuid/proxy/<build|or|service-name>/<port>/<requested-uri>. Where port is the port where the build service is running and requested-uri is the url that we want to request. The method used will be the same one of the current request.
• header with the Authorization headers
• CAPem which will hold the runner certificate.
• Subprotocols in order to be able to use websockets

Follow-up issue https://gitlab.com/gitlab-org/gitlab-ee/issues/11078