[GraphQL October 2021] Support @specifiedBy(url: "...") directive (#1000)

[ilslv]

Part of #1000

Synopsis

RFC

Solution

Add specified_by_url attribute to Scalar impl and derive macros.

Checklist

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[ilslv]

FCM

Support `@specifiedBy(url: "...")` directive via `specified_by_url` attribute argument in `#[graphql_scalar]` and `#[derive(GraphQLScalarValue)]` macros (#1003, #1000)

- support `isRepeatable` field on directives
- support `__Schema.description`, `__Type.specifiedByURL` and `__Directive.isRepeatable` fields in introspection

[tyranron]

@ilslv overall is good, but let's do some additional adjustments:

1. Check during proc macro expansion whether the given literal is valid URL via Url::parse().
2. Add codegen failure tests to cover the case where invalid URL is specified.

UPD: I also cannot grasp where the specifiedByUrl directive is inserted into __Schema. It's missed in the introspection query results, while the spec requires it being present. I do see where new_skip and new_include are used though.

[tyranron]

@ilslv there is also @deprecated directive is missing in __Schema when intospected. Please, fix in a separate PR.