provide means to extend ApolloDiagnostic

[lrlna]

For those building on top of apollo-compiler, it'd be useful to be able to create additional ApolloDiagnostic on top of the existing graphql validation diagnostics. This allows to have a unified way of providing validation + extenral graphql diagnostics

[goto-bus-stop]

Something I thought about briefly was that maybe DiagnosticData could be a trait instead of an enum, so users would be able to add their own implementations. Then ApolloDiagnostic would contain a Box<dyn DiagnosticData>. Ifff we can do that without imposing too much implementation burden on users.

[lrlna]

yea trait will quite possibly work. we'll just want to provide docs and a couple of examples

[goto-bus-stop]

I think the plan is gonna be to have a flexible diagnostic type in apollo-rs, that builds on ariadne's, that you can use to add messages/labels and that provides to_json() and get_line_column() and printing. Errors reported by apollo-rs would be convertible to this error type. Federation and other users could implement their own error types for their own use cases and also implement conversion to apollo-rs's generic diagnostic type for printing or use in JSON responses.

[lrlna]

@goto-bus-stop i feel like we've done a work for this already. shall we close it?