Provide pkgconfig via the CMake toolchain file on Windows

[glebm]

Is your feature request related to a problem? Please describe.

The Windows experience would be much nicer if vcpkg CMake toolchain file provided pkgconfig and set PKG_CONFIG_EXECUTABLE.

Proposed solution

Set PKG_CONFIG CMake variables in the CMake toolchain file.

Additional context

Currently users have to work around this by downloading and building pkgconfig themselves.
Internally, vcpkg already seems to have much of what would be needed to support it.

[JackBoosY]

We do need this.
cc @Neumann-A

[Neumann-A]

hmm added #17488 and #17487 so people can choose which one they want.

[JackBoosY]

@Neumann-A Also, we should output the usage of pkgconfig I think.