- rework `rocket_server` example as `simple`
- provide example in `GraphQLRequest` API docs
- mention GET query format in `GraphQLRequest` API docs and `simple` example
Additionally:
- fix `operationName` query parameter handling
- make `graphiql_source()` and `playground_source()` polymorphic over `subscriptions_endpoint_url` argument
- provide examples in `graphiql_source()` and `playground_source()` API docs
- move integration HTTP tests to a separate file
- test both sync and async `juniper_rocket` in integration HTTP tests
- polish `FromForm` unit tests
- set 1.62 Rust as MSRV
- use new fmt syntax where possible
- refactor `.to_owned()`, `.to_string()` and `.into()` usage
- rename `IntoResolvable::into()` as `IntoResolvable::into_resolvable()` to disambiguate with `Into::into()`
- use `#[derive(Default)]` for enums where possible
- use `bool::then_some()` where possible
