fix: PROJ initialization and network behavior

[brownag]

This PR addresses possible initialization inconsistencies in terra when bundled PROJ is used, and also adds projNetwork() function for controlling PROJ CDN usage. Intended to close #2001

• synchronizes PROJ and GDAL PROJ search paths by using both proj_context_set_search_paths() and OSRSetPROJSearchPaths() (when available)
• updates Windows-only conditional (.set_proj_search_paths(path) only when Sys.info()["sysname"] == "Windows") to set paths via projPaths(path, TRUE) whenever bundled PROJ data is detected in the package directory (as may be the case on macOS binary builds, for instance)
• adds projNetwork() for user control of PROJ CDN
  • exposes internal .PROJ_network, and updated to allow for getter-only behavior w/ enable=-1

This is a sample implementation/proof-of-concept. It passes CI checks on my fork. (I hope) it will help ensure that otherwise valid binary installations of terra with bundled PROJ work as expected, following on from #1996. Feel free to revise, change function names, cherry-pick, or disregard if I have made any errors here.