Support @specifiedBy for custom scalars

[stuebingerb]

From to the spec (1), https://spec.graphql.org/draft/#sec-Scalars:

When defining a custom scalar, GraphQL services should provide a scalar specification URL via the @specifiedBy directive or the specifiedByURL introspection field. This URL must link to a human-readable specification of the data format, serialization, and coercion rules for the scalar.

From the spec (2), https://spec.graphql.org/draft/#sec-Type-System.Directives.Built-in-Directives:

GraphQL implementations that support the type system definition language should provide the @specifiedBy directive if representing custom scalar definitions.

From the spec (3), https://spec.graphql.org/draft/#sec--specifiedBy:

directive @specifiedBy(url: String!) on SCALAR

Example:

scalar UUID @specifiedBy(url: "https://tools.ietf.org/html/rfc4122")
scalar URL @specifiedBy(url: "https://tools.ietf.org/html/rfc3986")
scalar DateTime
  @specifiedBy(url: "https://scalars.graphql.org/andimarek/date-time")

The @specifiedBy built-in directive is used within the type system definition language to provide a scalar specification URL for specifying the behavior of custom scalar types. The URL should point to a human-readable specification of the data format, serialization, and coercion rules. It must not appear on built-in scalar types.