add QEM schema to repo

[natestemen]

The development of a schema for quantum error mitigation techniques has been undergoing through an RFC, as well as a private repository. They are now at a point where they should be added, so we can start integrating them into the Calibrator and other tools.

I suggest creating a new directory mitiq/schema to add the files for the schema and work with them from there. More issues to come, but lets start simple.

cc @ecarlander @ruheenirodi comment here if you have any questions about getting these added, and let me know who I should assign to the issue.

[natestemen]

Oh also, please add a link to the repo if you're okay with making it public. If you'd like to keep it private that's okay, but I know others want to see!

[ecarlander]

We copied the contents out our working repo into the mitiq/schema directory, but if we should instead reserve that for more developed work we can switch back to that and I'll share the link! I also mentioned this in the PR I opened