create backends endpoint

[thomasmitchell]

backends endpoint should give:

• Some unique identifier representing the "version" of the data within. If this identifier changes, then there is new data for the backend
• Timestamp of last successful scrape
• Timestamp of last attempted scrape
• Health status of last attempted scrape
• Error message from last scrape, if failed
• Health status of last attempted auth
• Error message of last attempted auth, if failed