[Proposed change] Add support for schema metadata in introspection

[taion]

I would like to have a way to communicate schema metadata like a version or revision via the introspection mechanism.

We've found it useful for coordinating between the back end team and our front end teams to indicate the deployed version of the GraphQL schema and server in our dev environment.

Broadly, this seems like a more general thing. Currently we're setting this as the description on our query type, but that doesn't seem quite right.

The connection to things currently done by introspection are somewhat loose, but I think this satisfies how one would want such a feature to work decently well. "Real" clients should never query for fields like schema version per many discussions here around backward compatibility, but it can be quite convenient for developers to have some way to communicate what specifically is deployed within just the introspection mechanism.

[RemyRylan]

This would also be a great route for supplying validation info such as JSON Schema, string minLength, maxLength, etc... I currently force this into the description as well, but it would be so much more useful if it were standardized so that tools could be built around a common expectation of field metadata.

[taion]

Oh, wait, can we do this with schema directives?

[taion]

My bad. This is essentially a dupe of #300.