IDEA: Refactor Handlers by splitting artifacts generation and schema/executor generation

[ardatan]

Today handlers take a store and reads/writes that store while building a GraphQL Schema and an executor within getMeshStore
but instead handlers can have two methods;
getSource that returns MeshSource like the existing getMeshStore but it will take built artifacts as a parameter
getArtifacts that returns an array or a map that contains generated artifacts such as MySQL database introspection, gRPC protobuf descriptors, GraphQL schema for GraphQL sources etc.