introduce ValidatedExecutionArgs

[yaacovCR]

to safely export buildExecutionContext() as validateExecutionArgs()

motivation: this will allow us to:

1. export executeOperation() and executeSubscription() for those who would like to use directly.
2. add a perEventExecutor option to ExecutionArgs that allows one to pass a custom context for execution of each execution event, with the opportunity to clean up the context on return, a la Fix #894, add perEventContextResolver for subscribe #2485 and feat(subscribe): allow providing custom execute function for subscribe #3071, addressing Context lifecycle in subscriptions #894, which would not require re-coercion of variables.

The signature of the perEventExecutor option would be:

type SubscriptionEventExecutor = ( validatedExecutionArgs: ValidatedExecutionArgs): PromiseOrValue<ExecutionResult>

rather than:

type SubscriptionEventExecutor = ( executionArgs: ExecutionArgs): PromiseOrValue<ExecutionResult>

This might be a first step to integrating subscribe() completely into execute() (see: #3644) but is also a reasonable stopping place.

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[yaacovCR]

added motivation above.