Expose `Schema` class for runtime metadata

[timkendall]

We should implement the beginnings of schema definition support by code-generating a Schema class that exposes the following static properties seen in the example.

Runtime clients can use the metadata properties provided for instrumenting themselves with runtime metrics.

Example:

import { Schema } from './my-api'

// get the derived semantic version of the schema
Schema.VERSION // "1.0.1"

// get the sha256 of the schema SDL
Schema.SHA256 // "de8bcbe25b742e187385061f9af26ce061122a87ef9272d1b7e1ddd3e75f1d5f"