Move some view components to SpeziViews

[bauer-andreas]

Problem

PR #62 introduced several view abstractions which might be beneficial for all Spezi frameworks. This includes the CaseIterablePicker view (based on the PickerValue type alias) and the FormHeader view for beautiful From/List headers,

Solution

Move these views to SpeziViews.

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct and Contributing Guidelines

[bauer-andreas]

CaseIterablePicker is now part of SpeziViews with the release 1.8.0