refactor: subscribe: introduce buildPerEventExecutionContext

[yaacovCR]

= introduces buildPerEventExecutionContext that creates an ExecutionContext for each subscribe event from the original ExecutionContext used to create the event stream
= subscribe now directly builds the ExecutionContext instead of relying on createSourceEventStream
= introduces createSourceEventStreamImpl and executeImpl functions that operate on the built ExecutionContext rather the ExecutionArgs
= subscribe calls the createSourceEventStreamImpl function on the original context and eventuallys calls executeImpl on the per event context created by buildEventExecutionContext.

Motivation:

1. remove unnecessary buildExecutionContext call, reducing duplicate work
2. paves the way for easily enhancing the buildPerEventExecutionContext to add a new perEventContextFactory argument to augment the context argument passed to resolvers as need per event.

depends on #3654

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[yaacovCR]

Not the best diagrams (fault my own) screenshotted from yEd:

Original:

New:

Note that per event, we are now no longer re-performing the portions of BuildExecutionContext that do not need to be reperformed.

[yaacovCR]

Note that these changes do imply that the *Impl functions could be methods on an Execution Instance or the need for some other abstraction.

But that is already implied by the passing of ExecutionContext to the majority of functions in the file.

These functions may be renamed/reorganized in later refactoring!

[yaacovCR]

Offline, @IvanGoncharov added a few notes:

1. The spec also indicates that execution context should be built only once for subscriptions in that coercion of variables, for instance, is described only once, not per event.
2. At the same time, when CreateSorceEventStream and ExecuteSubscriptionEvent are managed by two separate services, as suggested by the spec, coercion will end up happening per event. This PR cannot, should not, and does not change that, because the provided implementation of the full subscription operation utilizes only a single service, exposing CreateSourceEventStream and Execute to do so.

[yaacovCR]

I rebased this on top of the simpler changes introduced in #3654

[IvanGoncharov]

This change solves the real issue.

I don't like some of the solutions here like "executeImpl" or "per even context", but I can't propose any other solution that doesn't involve bigger refactoring.

P.S. I'm working on such bigger refactoring in parallel