introduce new builder: `HTTPBuilder`

The `HTTPBuilder` uses a server process to do the actual compilation of the
just-in-time compiled Python modules. For client-server communication, the
established HTTP protocol is used.

Having a separate build process allows to run several Python programs using the
same `dune-py` module. Any conflicts are managed by the single server process
performing the actual build.
1 job for feature/httpbuilder in 14 minutes and 44 seconds (queued for 1 second)
Status Name Job ID Coverage
passed debian:9 gcc #24830