Upload meta-data to uniquely identify background mixing parameters

**Is your feature request related to a problem? Please describe.**
For background mixing, we need to specify background source file, frequency and skip variable. We should store these as metadata instead of trying to encode all information in file name.

**Describe the solution you'd like**
We would like to have metadata field that points to file name, frequency and skip variables
https://eicweb.phy.anl.gov/EIC/campaigns/datasets/-/blob/add-background-filename-to-config-data/config_data/egas_freq.csv?ref_type=heads

What other metadata will be useful to store?