Persisted queries

[shoooe]

Is your feature request related to a problem? Please describe.
Persisted queries aim at:

• Improve the performance of a GraphQL server
• Whitelist specific queries
• Reduce bandwidth usage

Describe the solution you'd like
Something similar to HotChocolate's persisted queries.

Describe alternatives you've considered
I considered implementing a layer, between the HTTP request and the Juniper engine, that could convert an ID into a GraphQL query stored within the executable.

That would solve the whitelisting problem and bandwidth usage but wouldn't improve the performance of the GraphQL server compared to just passing the query directly.

For that Juniper would have to expose the parsing and executing as two different function calls. That way I could parse the whitelisted queries at server startup and just reuse those parsed queries time and time again.

Additional context
Relay describes persisted queries.
Apollo also seems to have support for those.

[LegNeato]

We have an existing issue to split up the stages here: #726