feat: make haswidgets api also accept collections

Description

Adds 2 new API to HasWidgets:

[x] I have read the contribution guide: https://vaadin.com/docs/latest/contributing/overview
[x] I have added a description following the guideline.
[x] The issue is created in the corresponding repository and I have referenced it.
[x] I have added tests to ensure my change is effective and works as intended.
[x] New and existing tests are passing locally with my change.
[x] I have performed self-review and corrected misspellings.