Add support for parsing and validating a query without executing it

[ccbrown]

Currently there's no way to know if a request contains a subscription operation until you try to execute it. This is a problem for example in our graphql-ws implementation, which currently just has to invoke both juniper::execute and juniper::resolve_into_stream and see what works:

juniper/juniper_graphql_ws/src/lib.rs

Lines 301 to 303 in 84c9720

	// TODO: This could be made more efficient if juniper exposed functionality to allow us to
	// parse and validate the query, determine whether it's a subscription, and then execute
	// it. For now, the query gets parsed and validated twice.

It would be useful if juniper had a lower level API that could be used to parse and validate a query, determine what type of operation is requested, then execute it.

[LegNeato]

This would probably get rid of duplicate code between execute and execute_sync too.

[zacck-zz]

Hey @LegNeato I would love to take this on, but at the moment I have no idea where to get started, is it ok to ask for instructions on what needs to be done? Thank you so much!