Expose `data-view-only` attribute to WordPress data store

## Feature Description

In our current setup, we extract the `data-view-only` attribute from a DOM element and pass it into the React context (`ViewContext`). However, this method doesn't fully leverage the capabilities of the WordPress data store, leading to inconsistently using the `isAuthenticated` selector to check the view-only user.

The purpose of this issue is to enhance our WordPress data store by creating a new selector that exposes the `data-view-only` attribute. Additionally, it involves refactoring instances where the `isAuthenticated()` method is currently used to check the view-only user, replacing it with this new approach.

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## Acceptance criteria

* A new selector, `isViewOnly,` is implemented in the `core/user` data store, accurately reflecting the state of the `data-view-only` attribute.
* A `setIsViewOnly` action is created and appropriately dispatched in the entry files to set the initial view-only value in the `core/user` data store.
* All instances in the data store where `isAuthenticated()` is currently used to check the view-only user are successfully refactored to use the `isViewOnly` selector.

## Implementation Brief

* Finish and merge this POC [branch](https://github.com/google/site-kit-wp/compare/poc/expose-view-only-to-wp-data).
* Find all instances in the data store where `isAuthenticated()` is used to check the view-only users should be refactored to use the `isViewOnly` selector.

### Test Coverage

* Add test cases for the new `isViewOnly` selector.
* Fix any broken tests for the changes to replace the `isAuthenticated` selector.

## QA Brief

*

## Changelog entry

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