fetchKSSL: Fix for "status was 'SSL connect error'"

[brownag]

Thanks to Ben Marshall @Kramdog for bringing this up / reminding me that the SSL verification was still an issue in some cases. Since jsonlite changed behind the scenes, we need to attend to fetchKSSL-related downloads.

This updates fetchKSSL() to use {curl} internally for downloading the gzipped JSON from SoilWeb. This was what was done implicitly w/ fromJSON() w/ jsonlite <1.8.1, but with the ssl_verifyhost option set in the header.

While the base R approach / default headers included in jsonlite >1.8.1 work fine on non-gov machines we occasionally encounter errors related to SSL verification when in the office or connected to VPN, i.e.

Since we recently set up {curl}-based approach for replacing download.file() (due to timeout and SSL limitations), we just need to use the standard soilDB curl handle (including ssl_verifyhost=0) to sort this out. The gzipped json is downloaded to a temporary file with curl, then read as a gzfile() connection into jsonlite::fromJSON(), and then the temp file is unlinked

[brownag]

The broken test (https://github.com/ncss-tech/soilDB/actions/runs/3184898261/jobs/5193839591) in R devel is unrelated to the changes. See #265

[dylanbeaudette]

Looks good to me, thanks!