ARROW-12283: [R] Bindings for basic type convert functions in dplyr verbs

[ianmcook]

This adds support for as.character(), as.double(), as.integer(), bit64::as.integer64(), as.logical(), and as.numeric() in dplyr

[nealrichardson]

On the one hand, this is helpful and if nothing else may prevent some cryptic errors. On the other hand, it's a little inconsistent because they don't actually yield character (e.g.) vectors, and I wonder if that will cause other unexpected behavior. It's also different (FWIW) from the same methods on Array defined outside of dplyr (which may or may not be very useful): https://github.com/apache/arrow/blob/master/r/R/arrow-datum.R#L138-L145

Do dbplyr backends typically include these?

[ianmcook]

I think these are important for basic usability of any dplyr backend. dbplyr defines them for all of the major databases (https://github.com/tidyverse/dbplyr/search?q=%22as.numeric%22+backend) and many other dplyr backends do too. I hear your qualms about mixing R and Arrow paradigms, but I think practically speaking these are too important not to implement.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-12283

[ianmcook]

I created ARROW-12292 for us to reconsider whether the ArrowDatum methods of the same names should perhaps behave differently