[R] Implement as.integer and as.numeric for timestamp types etc. in Arrow dplyr queries

[asfimport]

In R, the POSIXct type are converted to seconds, so division and rounding are required within arrow, depending on the unit.

Reporter: SHIMA Tatsuya / @eitsupi

_{Note: This issue was originally created as ARROW-17428. Please see the migration documentation for further details.}

[github-actions]

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