[Backfill corrections] Convert dates-as-strings to Date types

[nmdefries]

Description

Support new input data format. After reading in data from a parquet file, convert time_value and issue_date fields from string to Dates.

Extend support of passing column names as args. Date conversions are applied to columns that are referred to in run_backfill() with col name variables. main() and functions that main() calls currently reference these with hard-coded names. Modify main and check functions to accept and use custom col names for issue_date, lag, and time_value fields.

Changelog

• NAMESPACE
• main.R
• utils.R

[jingjtang]

LGTM

[nmdefries]

@krivard This is ready to merge