Add libpq within a DESCRIPTION.SystemRequirements

[benlake]

Add libpq within a DESCRIPTION.SystemRequirements entry to provide tools a chance to discover RPostgreSQL has a system dependency, and to suggest the OS specific packages to install; for instance the renv dependency manager.

Some references to this usage in the community:

• https://r-pkgs.org/description.html#other-fields
• https://rstudio.github.io/renv/reference/sysreqs.html (See Details section)

Example specification in a package: https://crandb.r-pkg.org/igraph

I'm new to the R community and have mainly been focused on packaging R apps in various forms, so I'm sensitive to the quagmire that is the R communities' OS dependency woes. I also see a number of issues against this repository about a failure to have libpq installed, so hopefully this addition can help the tool-chains do the needful.

I'm not sure how tightly this R package is tied to the libpq versions, but if there is a desire to notate a version, I'm happy to add; e.g. (>= 10) or 14, or?

[glin]

+1, this would help a lot with Linux users in particular, who usually have to compile from source or install a precompiled binary from somewhere outside of CRAN. Or for users of the GitHub Actions for R, where the GHA workflow can automatically install system dependencies for packages that declare SystemRequirements.

As another example, RPostgres has:

SystemRequirements:	libpq >= 9.0: libpq-dev (deb) or postgresql-devel (rpm)