Add optional option for 'fre catalog builder'

[ceblanton]

Is your feature request related to a problem? Please describe.
fre catalog builder has new capability to determine standard_name vocabulary through a lookup table (uses variable and realm) to support MDTF.

Some variables are not in the lookup table and standard_name for those variables are still desired by MDTF. Additionally, we will soon need functionality to map multiple static variables to a single NetCDF file.

Both these capabilities require opening the NetCDF file, which is slow. Early examples of this approach are available in CatalogBuilder now as an optional argument to catalogbuilder.scripts.gen_intake_gfdl (https://github.com/NOAA-GFDL/CatalogBuilder/blob/main/catalogbuilder/scripts/gen_intake_gfdl.py#L32).

Describe the solution you'd like
Add the optional --slow option (default off) to the fre catalog builder tool.

Describe alternatives you've considered
Catalog testers/developers could use the catalogbuilder outside of fre-cli to exercise the --slow option.

Additional context
Add any other context or screenshots about the feature request here.