arrow

ARROW-11736: [R] Allow string compute functions to be optional

To test this locally, I added `-DARROW_WITH_UTF8PROC=OFF` to my cmake (it is`ON` by default), rebuilt the C++ library, and looked to see what tests failed. Then added the skip_if_not_available condition based on what function could not be found. Because the Arrow C++ compute functions are all accessed through the `CallFunction` interface, referenced by (string) name, there (currently) are no C++ bindings in the R package that require utf8proc, so there is no `#ifdef` and other conditional C++ build wrapping required.

`re2` will be the same, but we currently don't have any dplyr bindings for re2 functions. ARROW-11513 will add the first one, now that ARROW-10306 has merged.

To test this build configuration, I extended the `test-r-minimal-build` crossbow job to turn off these features. We should check the test output to confirm that the build indeed excludes utf8proc and re2 (and that the test suite still passes).

Closes #9809 from nealrichardson/r-without-strings

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