lava-callback need to be renamed

Over the time role of HTTP endpoint in kernelci-pipeline evolved from just LAVA callback (used to translate results from LAVA to KernelCI format), to more broad HTTP application that provide API to kci-dev.
I believe it should be renamed to something more generic and less misleading. Probably API will be misleading too, as we have kernelci-api, them maybe we can rename it to http.py (HTTP endpoint)?

Search the repository for the lava-callback name and inspect the HTTP endpoint entry point before choosing the replacement name. Rename the endpoint consistently, update its references, and verify that no old-name references remain.

Written by the indexing model from the issue text.