[R] Add snappy to Rtools Windows builds

[asfimport]

Followup to ARROW-3758 / #4622. The rtools builds of the C++ library for R Windows do not contain Snappy, which is very commonly used with Parquet files. 

r-windows/rtools-backports#6 is an attempt to build snappy with Rtools. Assuming that can be worked out, adding it to ci/PKGBUILD and setting ARROW_WITH_SNAPPY=ON there may be enough. 

Once this works, we should remove the parquet test skip that PR#4622 added.

Reporter: Neal Richardson / @nealrichardson
Assignee: Neal Richardson / @nealrichardson

PRs and other links:

• GitHub Pull Request #4681

_{Note: This issue was originally created as ARROW-5683. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 4681
#4681