Move arrow to Suggests to appease CRAN

[thisisnic]

Related to https://github.com/apache/arrow/issues/39806. Since it's not clear that we'll be able to resolve everything with CRAN before February 9, this PR moves arrow from Imports to Suggests. This will prevent your checks failing on CRAN due to arrow's potential archival, but if we don’t get removed, or when we do get back on CRAN, you can revert this.