populate initial with initialExpression result collection

[parthfloyd]

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

Fixes #2750

Description
Updated populateInitialValue() function in ResourceMapper.kt of the SDC module to populate questionnaireItem.initial with values of collection instead of just the first value of the collection.

Alternative(s) considered
None | Looking forward to feedback

Type
Bug fix

Screenshots (if applicable)

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[LZRS]

To support multiple initially selected options from the dropdown, not quite sure populating the Questionnaire.item.initial would work.
From this Questionnaire.item.initial invariant

que-11 Rule If one or more answerOption is present, initial cannot be present. Use answerOption.initialSelected instead answerOption.empty() or initial.empty()

I would suggest maybe using Questionnaire.item.answerOption.initialSelected instead

[parthfloyd]

@LZRS Thank you so much for your feedback!
I've pushed code & now it's conforming with the invariant rules. Please feel free if any changes or optimizations are required in the code.

Invariant:

que-8 Rule Initial values can't be specified for groups or display items

que-13 Rule Can only have multiple initial values for repeating items

que-11 Rule If one or more answerOption is present, initial cannot be present. Use answerOption.initialSelected instead

cc: @jingtang10 , @icrc-jofrancisco

[LZRS]

LGTM