[feature] conan v2 tools: Expose cppstd_flag function

[SpaceIm]

• I've read the CONTRIBUTING guide.

This helper is useful in recipes relying on unusual build systems, in order to inject C++ standard flag in a robust way. It's used in boost recipe. In conan-io/conan-center-index#14415, I had to rely on AutotoolsToolchain(self).cppstd because I know from conan source code that AutotoolsToolchain populates this attribute with cppstd_flag(conanfile) when an AutotoolsToolchain instance is created.
Obviously it's not very elegant, weird because it's called outside of generate() actually, and even confusing since boost is not built with Autotools.