[mlir][ods] Support dialect specific content emission via hooks

Thus far we can only generate the same set of methods even for
operations in different dialects. This is problematic for dialects that
want to generate additional operation class methods programmatically,
e.g., a special builder method or attribute getter method. Apparently
we cannot update the OpDefinitionsGen backend every time when such
a need arises. So this CL introduces a hook into the OpDefinitionsGen
backend to allow dialects to emit additional methods and traits to
operation classes.

Differential Revision: https://reviews.llvm.org/D72514