[R] strftime bindings

[asfimport]

Create bindings for strftime, now that ARROW-13174 has been merged.

Reporter: Nicola Crane / @thisisnic

Related issues:

• [R] Add support for locale-specific day of week (and month of year?) returns from timestamp accessor functions (blocks)
• [R] Bindings for strftime (duplicates)
• [C++][Compute] Add strftime kernel (depends upon)

_{Note: This issue was originally created as ARROW-13666. Please see the migration documentation for further details.}

[asfimport]

Ian Cook / @ianmcook:
We should also see if it would be straightforward to also implement format_ISO8601() from lubridate, and maybe some of the format functions from the clock package: https://clock.r-lib.org/reference/index.html