Support generating typed filters for async-graphql based on entities

[chipsenkbeil]

Per discussion in async-graphql/async-graphql#373, GraphQL doesn't support unions in input types today. This means that the typed predicates that are unions can't be used directly. Instead, I'll need to be able to generate a unique input type to handle all conditions for a given ent.

enum Predicate {
    Equals(...),
    GreaterThan(...),
}

// becomes

#[derive(async_graphql::InputObject)]
struct Predicate {
    equals: Option<...>,
    greater_than: Option<...>,
}

The challenge is that we'd need to make.a unique struct per type: String, Int, Float, Boolean, and ID. This isn't too demanding and could potentially be converted into a Custom Scalar as it supports maps (BTrees of names and values), lists (vec of values), and other types as defined here: https://github.com/async-graphql/async-graphql/blob/master/value/src/lib.rs#L123

If we can get away with making value be a scalar, then we could make custom predicate & filter input types that flatten the enums into structs of options. We'd want to give them names like GraphQLPredicate and GraphQLFilter (and GraphQLQuery).

In addition, we could still generate unique typed predicates for each of the five standard primitives of GraphQL.

Finally, we could add a derive of EntAsyncGraphQL and EntAsyncGraphQLFilter that would provide an object impl using the async_graphql macro and an input type filter that is a more specific MyEntGraphQLFilter where the edge and field are replaced with the graphql equivalents.

[chipsenkbeil]

Couple of examples of how filters are done in different implementations supporting GraphQL:

• https://dgraph.io/docs/graphql/schema/search/
  • Generates queryPost(filter: { datePublished: { ge: "2020-06-15" }}) where the field takes an object that is the predicate
  • No logic (or/and/not/...) specified
• https://grandstack.io/docs/graphql-filtering/
  • Generates Movie(filter: { year_lt: 1920 }) where the field and check are combined like year_lt
  • Supports logic filter: { OR: [{ year_lt: 1920 }, { title_contains: "River Runs" }] }