[mlir] Add an out-of-tree dialect example

This adds a minimal out-of-tree dialect template which can be used to start work on a standalone dialect implementation without having to integrate it in the main LLVM tree.

It mostly sets up the directory structure and provides CMakeLists.txt files to build a dialect library, an opt-like tool to operate on that dialect as well as tests. It could be expanded in the future to add examples of more user-defined operations, types, attributes, generated enums, transforms, etc. and linked to a tutorial.

Differential Revision: https://reviews.llvm.org/D77133