RFC: simplify subgraph getting started and federation

[nullxone]

Simplify subgraph getting started and federation

Continuing with the theme of designing around primitives, and optimising for a user getting started going from scratch to an API with only CLI commands, the following changes are related to subgraphs.

Rendered: https://github.com/hasura/graphql-engine/blob/ak/rfc/20240620-simplify-subgraph-getting-started-and-federation/v3/rfcs/dx/20240620-simplify-subgraph-getting-started-and-federation.md

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