[mlir][capi] Add DialectRegistry to MLIR C-API

Exposes mlir::DialectRegistry to the C API as MlirDialectRegistry along with
helper functions. A hook has been added to MlirDialectHandle that inserts
the dialect into a registry.

A future possible change is removing mlirDialectHandleRegisterDialect in
favor of using mlirDialectHandleInsertDialect, which it is now implemented with.

Differential Revision: https://reviews.llvm.org/D118293