Bring back solmate submodule

In https://github.com/Hats-Protocol/hats-protocol/commit/e23abebfbf1fd8eae30499b515d73b393da48562, we removed the solmate submodule in favor of storing select contracts in lib. This was done primarily to avoid errors with github actions observed in immediately previous pushes and PRs.

This issue would bring back the solmate submodule (assuming the problems with github actions can be addressed).