Support Parquet as an option for output flat views

[bashir2]

When we implemented the support for ViewDefinition (#821 and #916) we added relational tables as the only supported output (#822). We should extend this functionality and add an option for Parquet files as well. Note this is different from our default Parquet approach where we export full FHIR resources into the "wide" schema. Here, we create the flat schema of the given ViewDefinition in the output Parquet; so it is like a materialized view.

[bashir2]

This is now done; thanks @itsiggs.

[bashir2]

These are all the related PRs for posterity:

• ViewDefinition-to-Parquet Implementation #1130
• Incremental Run Support for View-to-Parquet #1158
• Trying Batch+Incremental E2E Changes on Clean Branch #1169
• Added Caching for View-to-Parquet writes #1171