Updated the code to throw expeption when main thread is blocked.

[aditya-07]

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

Fixes #1681

Description

1. Updated all runBlocking calls to runBlockingOrThrowMainThreadException that throws BlockingMainThreadException when called from main thread.
2. Marked all classes except for FhirOperator as internal as FhirOperator is supposed to be library's only public interface for developer applications.
3. Added a note regarding the BlockingMainThreadException to the functions in FhirOperator.
4. Updated tests to be called from worker thread using runBlockingOnWorkerThread and added some new tests as well.

Alternative(s) considered
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Type
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Checklist

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• I have read the Contributing page.
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• 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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[omarismail94]

Nice change Aditya! Left a couple of comments

[omarismail94]

This looks great @aditya-07 ! just one minor nit