feat: default observability client configuration

[mcollovati]

Description

When a custom observability client is not provided, settings are read directly from the agent configuration. It is possible to turn on and off the out-of-the-box frontend instrumentation, as well as completely disable the frontend observability.

Closes #196

Type of change

• [ ] Bugfix
• [x] Feature

Checklist

• [x] I have read the contribution guide: https://vaadin.com/docs/latest/guide/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.
• [ ] I have performed self-review and corrected misspellings.

Additional for Feature type of change

• [ ] Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.