[libsodium] Add pkgconfig file

[Adela0814]

For #19317
export the .pc file.
Tested usage on x64-windows via:

find_package(PkgConfig REQUIRED)
pkg_check_modules(libsodium REQUIRED IMPORTED_TARGET libsodium)
target_link_libraries(CMakeTest PkgConfig::libsodium)

• Changes comply with the maintainer guide
• SHA512s are updated for each updated download
• The "supports" clause reflects platforms that may be fixed by this new version
• Any fixed CI baseline entries are removed from that file.
• Any patches that are no longer applied are deleted from the port's directory.
• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• Only one version is added to each modified port's versions file.