Create an official python client to the GraphQL API

[dbendelman]

Please consider adding an officially maintained and documented python client for the graphql endpoint, e.g.

from dagster_graphql import DagsterGraphQLClient

client = DagsterGraphQLClient(host='localhost', port=9000)
client.launch_pipeline('my_pipeline', config={})

There is already code in the dagster_graphql test suite that implements something akin to a client (example), but all this code is tightly coupled with fairly complex test scaffolding and of course it's not easily usable outside that test suite.

Such a client would be incredibly useful:

• Pipeline solids launching other pipelines (our particular use case, and a common one from what I see in dagster slack).
• Easily interact with a dagit server from steps in a CI/CD pipeline.
• Pave the way for third party control planes for dagit to be written.
• Simplify dagster's own tests.

[mgasner]

should have documented oauth support

[sidkmenon-zz]

Should be closed with e929746