csv_populate_recordset and csv_agg

Hello hackers,

The `json_populate_recordset` and `json_agg` functions allow systems to
process/generate json directly on the database. This "cut outs the middle
tier"[1] and notably reduces the complexity of web applications.

CSV processing is also a common use case and PostgreSQL has the COPY ..
FROM .. CSV form but COPY is not compatible with libpq pipeline mode and
the interface is clunkier to use.

I propose to include two new functions:

- csv_populate_recordset ( base anyelement, from_csv text )
- csv_agg ( anyelement )

I would gladly implement these if it sounds like a good idea.

I see there's already some code that deals with CSV on

- src/backend/commands/copyfromparse.c(CopyReadAttributesCSV)
- src/fe_utils/print.c(csv_print_field)
- src/backend/utils/error/csvlog(write_csvlog)

So perhaps a new csv module could benefit the codebase as well.

Best regards,
Steve

[1]: https://www.crunchydata.com/blog/generating-json-directly-from-postgres