Explore apollo-rs AST -> graphql-core AST conversion

[erikwrede]

After parsing and validation in apollo-rs, we want to convert the AST back to the python format so we don't have to parse the query again to build the pyhton execution context. The conversion should happen as fast as possible to reduce overhead.

Test the following two variants:

• [x] Mirror the python AST classes and monkey-patch the isinstance checks
• [x] Import the GQL-Core classes and instantiate them (Test for GIL Overhead!)

[erikwrede]

Mirroring seems to be much faster, but only works by patching the __class__ attribute to get gql-core's isinstance checks to work:

#[pymethods]
impl OperationDefinitionNode {
    #[getter(__class__)]
    pub fn __class__(&self, py: Python<'_>) -> PyResult<PyObject> {
        let field_node = py.import("graphql.language.ast")?.getattr("OperationDefinitionNode")?;
        Ok(field_node.into())
    }
}