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ARROW-14821: [R] Implement bindings for lubridate's floor_date, ceiling_date, and round_date (#12154)

This patch provides dplyr bindings to for lubridate functions `floor_date()`, `ceiling_date()`, and `round_date()`. This is my first attempt at writing a patch, so my apologies if I've made any errors 🙂 

### Supported functionality:

- Allows rounding to integer multiples of common time units (second, minutes, days, etc)
- Mirrors the lubridate syntax allowing fractional units such as `unit = .001 seconds` as an alias for `unit = 1 millisecond`
- Allows partial matching of date units based on first three characters: e.g. `sec`, `second`, `seconds` all match `second`
- Mirrors lubridate in throwing errors when unit exceeds thresholds: 60 seconds, 60 minutes, 24 hours 

~~### Major problems not yet addressed:~~

~~- Does not yet support the `week_start` argument, and implicitly fixes `week_start = 4`~~
~~- Does not yet mirror lubridate handling of timezones~~

~~I'd prefer to fix these two issues before merging, but I'm uncertain how best to handle them. Any advice would be appreciated!~~
 
~~### Minor things not yet addressed~~

~~- During rounding lubridate sometimes coerces Date objects to POSIXct. This is not mirrored in the arrow bindings: date32 classes remain date32 classes. This introduces minor differences in rounding in some cases~~
~~- Does not yet support the `change_on_boundary` argument to `ceiling_date()`. It's a small discrepancy, but it means that the default behaviour of the arrow dplyr binding mirrors lubridate prior to v1.6.0~~

EDIT: issues now addressed! 


Authored-by: Danielle Navarro <djnavarro@protonmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>