Fix: Issue in the extension URL extracted using profile loader

[vbothe23]

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

Fixes #[issue number] #2029

Description Issue: When using structure definition based extraction with ProfileLoader, the questionnaireItem definition URL was added in the extension instead of the profile URL.

Fix: When adding custom definition based extension to the resource in the ResourceMapper, the extension URL is now fetched the ElementDefinition in the given profile.

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

Type Bug fix

Screenshots (if applicable)

Checklist

• [x] I have read and acknowledged the Code of conduct.
• [x] I have read the Contributing page.
• [x] 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.
• [x] 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.
• [x] 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).