feat(juniper_codegen): add conversions for enum variants to strings

[garbados]

Hi!

This PR expands the GraphQLEnum derive macro to provide conversions from enum variants into strings. Currently, the macro only provides conversions for InputValue, but in my usage it became necessary to map enum variants into Value::scalar::<String> types so that they can be used in GraphQL responses, specifically as the second parameter of FieldError::new. Mapping enum variants to strings directly, rather than a Juniper type, is an artifact of my not knowing much.

I'm new to Rust and even newer to Juniper, so this PR is intended for review and discussion, and I am happy to incorporate any feedback.

[LegNeato]

Hello! Thank you for the PR and welcome! 🍻

First, I think there are some legit errors on CI related to this PR:

https://dev.azure.com/graphql-rust/GraphQL%20Rust/_build/results?buildId=684&view=logs&j=150c9611-1cfc-59c0-d4e8-92df330f7380&t=10bef84d-ad0f-569a-c6cb-c54604a7beb6&l=458

(sorry, our CI is a bit messy right now...I am working to clean it up).

Second, I think this sounds like a great change but need to think about it a bit more. I haven't really touched enums too much 😄

[garbados]

First, I think there are some legit errors on CI related to this PR:

I see those errors but none are related to the code I added, so I don't know how to make sense of them . Can you provide any guidance?