[FEA] Support pattern as a vector for ToUnixTimestamp, UnixTimestamp, FromUnixTimestamp and possibly other operators if any

[razajafri]

Is your feature request related to a problem? Please describe.
Cudf supports only strf patterns while spark supports Java date/time patterns defined in SimpleDateFormat as of Spark 3.0. We have a utility function in DateUtils that converts a Java date/time pattern literal to strf pattern. We need a way to support patterns that are vectors

Describe the solution you'd like
It may be possible for us to write a unary transform to convert a Java date/time vector to a strf pattern vector

Describe alternatives you've considered
Currently there are no viable alternatives, unless cudf decides to support Java patterns.

[jlowe]

We need a way to support patterns that are vectors

To clarify, the pattern vector is a column of patterns that has the same row count as the string column to be parsed, i.e.: each row can have a custom format string describing how the timestamp should be parsed.