Add FhirEngine interface method 'withTransaction'

[LZRS]

IMPORTANT: All PRs must be linked to an issue (except for extremely trivial and straightforward changes).

Fixes #2531

Description Support performing FhirEngine actions as a single atomic transaction

Alternative(s) considered Have you considered any alternatives? And if so, why have you chosen the approach in this PR?

Type Choose one: (Bug fix | Feature | Documentation | Testing | Code health | Builds | Releases | Other)

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Checklist

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• [x] I have signed the Google Individual CLA, or I am covered by my company's Corporate CLA.
• [ ] I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
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• [x] I have built and run the demo app(s) to verify my change fixes the issue and/or does not break the demo app(s).