[FEA] Support wider range of date/time formats with unix_timestamp

[andygrove]

Is your feature request related to a problem? Please describe.
Once #1113 is merged, we support a small number of date/time formats on GPU in unix_timestamp / to_unix_timestamp based on a list of supported formats.

It would be better to add logic to determine if a format is supported so that we can support a wider range of formats. For example, we know that we can't currently support .SSS on GPU because cuDF and Spark have different support for variable number of milliseconds. There may be other cases like this.

Describe the solution you'd like
Detemine whether a format is supported based on the invidial parts of the format.