Backport cppstd_flag from Conan 2.x

[uilianries]

Related to the PR #15710, this PR backports the method available in Conan 2.x

In Conan 1.x we have cppstd_flag available already, but it receives settings only is under conans.tools.

This change creates a wrapper to make it available under conan.tools.build and should pass conanfile instead.

Changelog: Feature: Promote cppstd_flag in the new conan.tools.build module.

Docs: conan-io/docs#3602

• Refer to the issue that supports this Pull Request.
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.