Added support to search using multiple resource types

[aditya-07]

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

Fixes #611

Description
Added a nested search to the current Search class so that the user can do a search reverse chained search like
Return all Patients who have a Condition - Diabetes referenced by Condition.subject property

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

Type
Choose one: Feature

Screenshots (if applicable)

Checklist

• I have read and acknowledged the Code of conduct
• I have read How to Contribute
• I have read the Developer's guide
• 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
• I have run ./gradlew spotlessApply and ./gradlew spotlessCheck to check my code follows the style guide of this project
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• I have built and run the reference app(s) to verify my change fixes the issue and/or does not break the reference app(s)

[codecov-commenter]

Codecov Report

Merging #697 (b1d26a3) into master (6db0990) will increase coverage by 1.10%.
The diff coverage is 78.37%.

❗ Current head b1d26a3 differs from pull request most recent head 5c0a35e. Consider uploading reports for the commit 5c0a35e to get more accurate results

@@             Coverage Diff              @@
##             master     #697      +/-   ##
============================================
+ Coverage     52.80%   53.90%   +1.10%     
- Complexity      196      198       +2     
============================================
  Files            47       48       +1     
  Lines           981     1011      +30     
  Branches        136      139       +3     
============================================
+ Hits            518      545      +27     
- Misses          380      381       +1     
- Partials         83       85       +2     

Impacted Files	Coverage Δ
...n/java/com/google/android/fhir/search/SearchDsl.kt	67.16% <50.00%> (+2.54%)	⬆️
.../java/com/google/android/fhir/search/MoreSearch.kt	71.49% <76.00%> (+1.04%)	⬆️
...ava/com/google/android/fhir/search/NestedSearch.kt	90.00% <90.00%> (ø)

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[jingtang10]

Great job!!!

[jingtang10]

QQ: what happens when you try to have mulit-level nesting? Have you tested? Can you add a test case?

[jingtang10]

QQ: what happens when you try to have mulit-level nesting? Have you tested? Can you add a test case?

[aditya-07]

QQ: what happens when you try to have mulit-level nesting? Have you tested? Can you add a test case?

Added search_practitioner_has_patient_has_conditions_diabetes_and_hypertension test which handles multiple mulit-level nestings.

[jingtang10]

please refactor so that count queries don't include sort order statement and limit statement? throw exception and add a test case as well please.

happy for you to do that in a follow-up immediately after this pr.