Allow users to select pre-loaded metadata mapping files

[tballison]

Metadata mapping is hard. Even loading a csv adds an extra step. We can either pre-load the digipres or let users pick a digipres-default or standard-default.

[tballison]

For now, let's load this automatically only for the csv output. We can add this functionality later to the other emitters.

The challenge is that the naive user probably expects different behavior for different types of output.

Filesystem: probably better without defaults (just write everything)
OpenSearch: probably better without defaults (just write everything) for the naive user.
JDBC: needs metadata mappings (there is no naive user using jdbc...lol)