Support additional format specifiers in from_timestamps

[davidwendt]

Reference #5991

This PR adds support for the following format specifiers in cudf::strings::from_timestamp

%a and %A -- weekday names (passed into the API)
%b and %B -- month names (passed into the API)
%u - ISO weekday (1-7)
%w - weekday (0-6)
%U - week of the year (Sunday based)
%W - week of the year (Monday based)
%V - ISO week of the year
%G - Year based on ISO weeks

This adds a new parameter to the API for the caller to pass then string names for the weekdays and months. These are only required if the %a, %b, %A, %B specifiers are contained in the format string.

The change to from_timestamps is mainly a rewrite to include logic for these specifiers. Some common code required corresponding changes to to_timestamps and is_timestamps though these functions have not changed in this PR.

[codecov]

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[devavret]

LGTM

[codereport]

Looks really good!

[davidwendt]

@gpucibot merge