[mlir][linalg] Add linalg_opdsl tool first draft.

* Mostly imported from experimental repo as-is with cosmetic changes.
* Temporarily left out emission code (for building ops at runtime) to keep review size down.
* Documentation and lit tests added fresh.
* Sample op library that represents current Linalg named ops included.

Differential Revision: https://reviews.llvm.org/D97995