Replace calls-gitaly instrumentation with a field extension [RUN AS-IF-FOSS]

What does this MR do?

In order to make our resolver tests play nicely with our instrumentation, I realised that the most sensible thing was to replace the instrumentation with field extensions. This has the additional benefit is that field extensions are the supported API for modifying field resolution. Significantly, field instrumentation is one of the proc-dependent features that are incompatible with the new interpreter (see: https://graphql-ruby.org/queries/interpreter.html#compatibility).

We intend to move to the interpreter (see: #210556 (closed) and !27536 (merged)) - and replacing the gitaly instrumentation will be required.

Related to: #210556 (closed)

Does this MR meet the acceptance criteria?

Conformity

As a developer facing change, with no user-visible modification of behavior, no changelog entry is needed.

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
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• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.