Implement endpoint to perform request to services running in builds

What does this MR do?

In order to implement https://gitlab.com/gitlab-org/gitlab-ee/issues/5276 we need several modifications and this is one of them.

In this MR:

• add a new endpoint to proxy websocket requests
• extend the JobsController controller to handle the proxy request calls
• add the feature flag build_service_proxy to allow/disallow service proxy connections

What are the relevant issue numbers?

Refs https://gitlab.com/gitlab-org/gitlab-ee/issues/5276 and closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9538

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer