[GQL] Support `#ready?` in resolvers

What does this MR do?

Fixes: #218289 (closed)

This adds support for resolvers to make use of #ready? in our GraphQL API. This is used in !32328 (merged)

There are a couple of other framework improvements here (matchers, the addition of BaseResolver#synchronized_object) that have been included in the interests of iteration and making MRs small and comprehensible.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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