Create FHIRFlat specification

[abhidg]

FHIRFlat is a Parquet based format that expresses FHIR resources in place of its native JSON Bundle export format.
Outcome of this is a versioned specification that maps each FHIR resource to a Parquet schema.

Note that we do not intend to cover all FHIR resources (about 150!), instead we will only focus on the resources that arise from https://github.com/globaldothealth/isaric3/issues/14.