Consider re-introducing lints to apollo-rs

[SimonSapin]

In apollo-compiler 0.11 validation returns a Vec that can contain:

• Any number of error-level diagnostics
• A warning for enum value definitions with non-uppercase names
• An "advice" for custom scalar definitions without an @specifiedBy directive

To find out if a document is valid, callers need to iterate the vec and find if any of the diagnostic return true for .is_error().

In apollo-compiler 1.0 (in beta as of this writing) the API changes so that the library does that filtering and returns Result::Err for invalid documents. However as discussed starting at #709 (comment) returning potential warnings and advice for a valid document makes the API awkward. Since they arguably don’t belong in validate() in the first place, they could be moved to a separate lint() method.

But with only two rules, this linter would not be very valuable and we don’t expect many users to go out of their way to call it explicitly.

An upcoming will therefore remove these two non-error rules from apollo-compiler entirely. This issue keeps a record of this regression of functionality. These two rule could be restored one day if we want to seriously work on having a GraphQL linter as part of apollo-rs.