[feature] Consider porting conans.tools.cppstd_flag to Conan 2.0

[uilianries]

Suggestion

Port conans.tools.cppstd_flag to conan.tools.build.cppstd_flag

Why?

Some projects use a limited build script which require our manual intervention, passing all needed compiler flags, including libcxx, cppstd, build_type, fPIC, ... For those cases, we need to honor the cppstd version from the profile with what will be used by the compiler.

As a real example from CCI: https://github.com/conan-io/conan-center-index/pull/13552/files#diff-a3929dbede89f0e2d22b994ee16d101290e2d32f05767c52cc580a4edf1d950eR198

As we need to translate cppstd to the compiler flag, cppstd_flag would help us.

• I've read the CONTRIBUTING guide.

[memsharded]

https://docs.conan.io/2/reference/tools/build.html#conan.tools.build.flags.cppstd_flag already there