Add variable Extension support

[shoaibmushtaq25]

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

Fixes #1059

Description
Clear and concise code change description.

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 the Contributing page.
• 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.
• I have run ./gradlew check and ./gradlew connectedCheck to test my changes locally.
• 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).

[codecov]

Codecov Report

Merging #1315 (64ba13c) into master (ae4c26a) will decrease coverage by 0.80%.
The diff coverage is 24.02%.

@@             Coverage Diff              @@
##             master    #1315      +/-   ##
============================================
- Coverage     84.50%   83.70%   -0.81%     
- Complexity      686      691       +5     
============================================
  Files           148      148              
  Lines         10697    10849     +152     
  Branches        832      861      +29     
============================================
+ Hits           9040     9081      +41     
- Misses         1244     1350     +106     
- Partials        413      418       +5     

Impacted Files	Coverage Δ
...iews/QuestionnaireItemDropDownViewHolderFactory.kt	65.71% <0.00%> (+14.28%)	⬆️
...android/fhir/datacapture/QuestionnaireViewModel.kt	54.46% <24.18%> (-25.43%)	⬇️

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