core (main): add a mechanism to have a shared temp directory: this directory may be used to share files between backend and frontends. Normally, an os-dependent temporary directory is created for that, but if this option is set, the directory will be created in <local_dir>/<cache_dir>, which may be useful in some use case (e.g. containerized frontends and backend).

[DEFAULT]
log_fmt = [%%(name)s] %%(message)s
media_dir = /src/libervia-media