LAVA callback handler

[gctucker]

LAVA jobs send their results via a callback mechanism. This is not directly compatible with the KernelCI API so an intermediate handler is needed to receive the callback HTTP requests, parse them and send new requests to the API.

• callback config added to kernelci.config.runtime.lava
• callback service added to receive the requests
• verified that the callback service can parse the results
• verified that the callback service can parse the logs
• verified locally that the results and logs can be sent to API and storage
• verified on staging

Note: This only covers plain text logs; for HTML support see #274