add dplyr translation support for TIMESTAMP_TRUNC

[era127]

The dplyr translation does not currently support the floor_date() function. Should it be mapped to the TIMESTAMP_TRUNC sql statement? The POSIXt objects are mapped to TIMESTAMP and not DATETIME so I think TIMESTAMP_TRUNC is best.

This is derived from dbplyr's translation for Postgres.

      floor_date = function(x, unit = "seconds") {
        unit <- arg_match(unit,
          c("second", "minute", "hour", "day", "week", "month", "quarter", "year")
        )
        sql_expr(TIMESTAMP_TRUNC(!!unit, !!x))
      },

I'm not sure how it would handle the translation of the timezone argument to TIMESTAMP_TRUNC because floor_date() does not support a timezone argument.

[hadley]

Good idea!

[hadley]

Docs here: https://cloud.google.com/bigquery/docs/reference/standard-sql/timestamp_functions#timestamp_trunc

[hadley]

Will wait on tidyverse/dbplyr#1357 and tackle a swathe of date functions all at once.