Implicit external check

[jclyons52]

Fixes #2443

As specified in the Federation v2 documentation The @external directive is no longer required for @key fields, but it is still required for @requires and @provides. When fields are part of the key they should implicitly be considered @external.

This PR updates the allFieldsAreExternal method on the entity to take a federation version parameter, if the verstion is set to 2 it checks if the relevant field is is listed in the @key directive. If it is, the field is considered external without the @external directive.

This PR also requires the resolvable argument to the @key directive to be set to false for it to consider any fields implicitly external. This is not entirely in line with the spec as it seems that if resolvable is false the entity should be excluded irrespective of what fields are external. In this PR, a less significant departure from the current behavior is favored over correctness as the change would potentially cause existing schemas to start failing.

This PR does not address reference types that have multiple keys defined. from the documentation it seems like a reference type should only use one key directive. All examples show entities with a single key, but the description is ambiguous:

A subgraph that references an entity without contributing any fields can include the fields of any @key in its stub definition

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

Coverage increased (+0.02%) to 75.66% when pulling 500195a on implicit-external-check into 5065163 on master.

[jclyons52]

This may also resolve #2387

[jclyons52]

Nevermind the lack of multi key support, that seems like it's broadly an issue that will have to be addressed #1031

[StevenACoffman]

Thanks so much! We would love any help on #1031 if you can!

[jclyons52]

Thanks so much! We would love any help on #1031 if you can!

No worries, I'll pick that up next 99time, which is every other Monday

[jdavisclark]

awesome. thanks for getting this fixed!