Reconsider which parts of execution/introspection to stabilize

[SimonSapin]

https://docs.rs/apollo-compiler/1.0.0-beta.24/apollo_compiler/execution/index.html exposes public APIs intended to cover most of a GraphQL execution pipeline, end to end from variable values coercion/validation to a full Response data structure. Router ended up not using some parts of this such as SchemaIntrospectionSplit, instead not (yet) supporting queries that mix schema-introspection fields and federated fields: apollographql/router#2789

For stabilization of apollo-compiler 1.0 I’d prefer to find a more minimal API surface that still exposes useful introspection functionality.