Improve custom_header.yaml management

[Zebreu]

custom_header.yaml is a file crucial to the software but is not read alongside the input data. Instead, it's copied into the docker image then reused for any request. We should have a mechanism that allows to offer one as an input, and at least we should have setup instructions: per deployment, make sure data mappings are accurate then move it to the local_data/ directory so that the Docker build can copy it.

Some of the code around the file also assumes some absolute paths and we'll need to be careful there.

[Zebreu]

The server endpoint "provide_files" enables this.