[mlir][CAPI] Introduce standard source layout for mlir-c dialect registration.

* Registers a small set of sample dialects.
* NFC with respect to existing C-API symbols but some headers have been moved down a level to the Dialect/ sub-directory.
* Adds an additional entry point per dialect that is needed for dynamic discovery/loading.
* See discussion: https://llvm.discourse.group/t/dialects-and-the-c-api/2306/16

Differential Revision: https://reviews.llvm.org/D94370