Rewrite examples to use public APIs

[retzkek]

Re-write all the examples to use public APIs so users have something concrete to learn with, like the Deutsche Bahn example I included in the plugin repo PR. Need to find an API that has numerical timeseries data.

[retzkek]

The GitHub API is probably a good candidate, with lots of timeseries data that's relatable to by anyone who wants to use this; while it needs authentication, once a user has authenticated to Grafana through GitHub OAuth the token can be passed through. I've started a couple dashboards that anyone with a GitHub account can view, without needing anything more than the basic scopes:
https://grafana.kmr.me/github-demo/d/N2Xxd_3Zz/security-advisories?orgId=1
https://grafana.kmr.me/github-demo/d/u6O_F_qWk/repository?orgId=1

Need to support passing ISO8601 timestamps per #10 to properly scope the requests.

[retzkek]

I've cleaned up the README but left the example queries, they illustrate the functionality well I think. I put the DB example in a wiki page: https://github.com/fifemon/graphql-datasource/wiki/Deutsche-Bahn-Arrivals

I'll write up the GitHub example in the wiki before closing this.

[retzkek]

The test source server and dashboard provide consistent examples that users can learn and experiment with locally.