Configure cynic schemas in Cargo.toml

[obmarg]

I'm not certain about this idea, but it could be nice for the names & locations of cynic schemas to be configured in the metadata section of the users Cargo.toml.

This would allow:

• The build.rs to read from here
• Potentially the derives could read from there (although tbh I'd rather they don't for speed purposes)
• The CLI could read from there for introspection
• If I integrate the generator into the CLI then it could also read from there
• Any CI integrations (probably powered by the CLI) could also read from there.

My main reservation is adding extra dependencies (either cargo_metadata or the toml library) into the dep tree. But if we end up supporting all the above use cases it could be pretty nice.

[obmarg]

An alternative to this might be to recommend a conventional place to put schemas, combined with a cynic specific config file. So:

- Cargo.toml
- schemas/
  - github.graphql
  - cynic-schemas.json

Or similar? Not sure

[obmarg]

Forgot I'd already documented this in #687 - but I prefer the description here so I'll close #687

[obmarg]

A sensible approach to this would be:

• Add a cynic-cargo-toml 3rd party library.
• This provides a function to call in build.rs that will:
  • Read schema paths out of Cargo.toml (or a separate file? not sure)
  • Register them as schemas (either with rkyv or not)
• The derive mechanism continues to work as usual.
• The CLI can be updated to allow easy introspection of the defined schemas as well.

A future extension could have the build.rs read URLs out of Cargo.toml and then introspect those, but I'm still skeptical that's a good idea.