Hard-code type cast map on front end

[seancolsen]

Stacked on #3819

As discussed today, the front end needs a way to know which types a column can be cast to. On that call, @Anish9901 and I had originally decided to build an API for this. However @Anish9901 later decided it would be better to hard-code this cast mapping on the front end, at least for now. This PR implements that hard-coding strategy, which allows the type of columns to be changed again.

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[pavish]

Looks great!