Add support for unitOption and/or unitValueSet

[williamito]

This is a continuation of #1550, see there for additional context.

A primary purpose of Questionnaire item type='quantity' is the ability for the user to select a unit from a list of options using the unitOption or unitValueSet extensions. We currently do not support either, so type='quantity' items are not very usable in SDC.

Describe the solution you'd like
Add support for unitOption or unitValueSet, where it should:

1. Show a unit picker (dropdown?) in the rendered Questionnaire
2. In the questionnareResponse, generate a valueQuantity
3. Populate the valueQuantity with any unit, system, code information provided in questionnaire-unit

[shelaghm]

Design mocks

[shelaghm]

Updated icon: squarefoot @jingtang10 @santosh-pingle