Simplify Adding New Endpoint Types

[nwmac]

• Currently to add a new endpoint type, you have to create a thin backend plugin
• Update the backend to allow a new endpoint type to be described in a yaml file (type, supported auths etc)
• Backend reads these endpoint type descriptions and adds them as types that can be registered, proxied etc
• Front-end developer can now easily add support for a new endpoint type without needing to touch the backend code

Example use case would be being able to add GitHub as an endpoint so that a user can then connect to GitHub with an access token - we can then allow users to deploy from private GitHub repositories from within the UI.

Could do the same for GitLab as well.

Richard can help with the UI changes if needed.