[feature] conan v2 tools: expose a public function like tools.msvs_toolset

[SpaceIm]

• I've read the CONTRIBUTING guide.

boost recipe needs to inject toolset version. In conan v1 there is tools.msvs_toolset. In conan v2 there is no public function. The closest function suitable for cross conan v1/v2 support is a private static method of MSBuildToolchain: _msvs_toolset. Current workaround in boost recipe is get this value from MSBuildToolchain(self).toolset since it's populated with value returned by _msvs_toolset(conanfile).