[graphiql] Example Loader Plugin

I suggest adding a `GraphiQLExampleLoaderPlugin`. `defaultTabs` is fine but when a GraphQL Schema is large tabs can quickly become too many and once you've edited a default tab there's no way to restore it to its default short of resetting all tabs.

An example-loader plugin could be 3rd party but having it provided along with the Explorer and Code Exporter could be nice because I think many folks would want to leverage an Example Loader plugin and to not need to reach for a 3rd party module has benefits.