Federated Mutations (orchestration framework)

Some of the usecases being considered as part of this issue:

- I want to issue multiple GQL queries/mutations and interleave business logic amongst them
- I want to apply validation logic before a mutation
- I want to add computed fields to my existing object types
- I want to run complex authorization logic before applying a mutation
- I want to run a saga/workflow of business logic interleaved with queries/mutations

No files, tests, or entry points are named. Start by reviewing the listed GraphQL orchestration use cases and clarifying the intended scope, then establish how validation, authorization, computed fields, and interleaved workflows should be covered before implementation; done requires an agreed design and corresponding tests.

Written by the indexing model from the issue text.