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GH-38138: [R] Add curl to suggests for use of `skip_if_offline()` (#38140)

### Rationale for this change

testthat now requires the curl package for `skip_if_offline()`. Previously tests were skipped; how the tests fail (although the message is somewhat bizarre).

### What changes are included in this PR?

Include curl explicitly as an optional dependency so that our dependency install step picks it up.

### Are these changes tested?

Covered by existing tests

### Are there any user-facing changes?

No.
* Closes: #38138

Authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Signed-off-by: Jacob Wujciak-Jens <jacob@wujciak.de>