Specify data endpoint when invoking $apply on a PlanDefinition

[divyaramnath-13]

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

Fixes #1981

Description
The data endpoint needs to be specified when invoking $apply on a PlanDefinition that contains an applicability condition represented as a CQL expression defined in a Library resource. If this is not set, the CQL logic will not be evaluated and the corresponding PlanDefinition action will not be executed.

Type
Choose one: Bug fix

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).

[divyaramnath-13]

Looping in @vitorpamplona

[vitorpamplona]

LGTM We might need to do the same work for the terminology and content endpoints, but maybe we should wait to see an issue first.