[extension] Add `(pkg_config ...)` field.

As suggested in https://github.com/ocaml/dune/pull/1886 , adding a field for automatically importing pkg_config build flags could be convenient and save quite a few boilerplate for C users.

The idea would be to have a new feature:

(using pkg_config)

which would extend the (library ...) stanza to add a field:

(pkg_config gtk+-3.0 >= 3.18)

Such field should unsugar to something similar to:

(rule
 (targets
   cflag-gtk+-3.0.sexp
   clink-gtk+-3.0.sexp)
 (action (run ./dune_config.exe -pkg gtk+-3.0 -version 3.18)))
...
 (c_flags         (:include cflag-gtk+-3.0.sexp))
 (c_library_flags (:include clink-gtk+-3.0.sexp))

where dune_config is the straightforward call to configurator [that could be embedded by dune avoiding the .exe and intermediate file of course]

Start by reading the linked pull request 1886 and the current handling of the library stanza and extension syntax. Trace how the proposed pkg_config field could invoke configurator or dune_config.exe and provide C flags. Done means the syntax, generated flags, and version constraint behavior are implemented and covered by appropriate tests.