Build and deploy native MinGW toolchain packages together with cross-compilation one

[Blackhex]

• Adds separate workflow, mingw-native-toolchain.yml, that builds native MSYS2 packages forming MinGW toolchain both using MSYS2 MinGW cross-compilation toolchain, or the native toolchain itself (built and deployed in the previous iteration).
• Adds the mingw-native-toolchain.yml workflow to main.yml workflow so the results can be deployed to MSYS2 packages repository.
• The build of cross-compilation toolchain can be skipped with build_cross parameter, the build of native toolchian can be skipped with build_native parameter, and whether native or cross-compilation toolchian is used to build native toolchain is controlled with build_native_with_native parameter.

[eukarpov]

Only minor comments after the first read. The CI looks good.