fst_tbl format

[wlandau]

Prework

• Read and abide by drake's code of conduct.
• Search for duplicates among the existing issues, both open and closed.

Proposal

As I believe @kendonB mentioned at one point, it would be nice if fst-based drake formats could accommodate tibbles. The current format = "fst" is not designed for this, but taking format = "fst_dt" as precedent, we could define a new format = "fst_tbl".