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Allow merging some cases of unordered directives (built-in) with ordered directives (custom) (#235)

Merging directive lists like `scalar Foo @bar @baz @biff` across service schemas can be unnecessarily arduous when the lists are nearly identical.
This change permits some cases to merge cleanly when they do not impact expected behavior.

Critically, the GraphQL spec says [directive order is significant](https://spec.graphql.org/October2021/#sec-Language.Directives.Directive-order-is-significant), so
this change goes through some extra merging pain in gateway to allow more flexible service schemas.

This is most obviously painful when a built-in directive is reused identically across service schemas, like so:

    scalar Foo @specifiedBy(url: "bar")
    scalar Foo @specifiedBy(url: "bar")

However, this change also permits custom directives if they are only in one of the services, or are identical.
For example, in the below list of service schemas, A+C and B+C are permitted to merge. Note that A+B cannot be merged because they do not have identical custom directives.

    scalar Foo @specifiedBy(url: "bar") @baz(biff: 1)
    scalar Foo @specifiedBy(url: "bar") @baz(biff: 2)
    scalar Foo @specifiedBy(url: "bar")

There are other edge cases due to significant ordering, but I've documented those on the relevant functions.


All changes:

* Allow merging slightly dissimilar directive lists
    - Allow non-empty with empty directive lists
    - Permit different sets of definitely-unordered directives. This allows one schema to add @specifiedBy and another a custom directive
* Improve error messages and docs
    - Clarify directive location merge docs
    - Extract helper to identify executable directive locations, improve error message clarity
    - Include mismatched directives in error
    - Fix invalid error message for scalar directive merge
    - Add type, field, and directive names for context on merge errors
* Fix merges for input objects

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