[DataViewsPicker]: DataViewsPicker.BulkActionToolbar now renders only the bulk-selection info and action buttons

[ntsekouras]

What?

Extracted from #77333 (comment)

While working in updating the revisions screens to use the picker-activity layout I noticed a few things we could improve around the exports and organization of the footer components.

These changes will help more use cases and enable better composition and some more will be explored separately when we'll try to integrate this layout in GS revisions.

This PR extracts:

1. In trunk DataViewsPicker.BulkActionToolbar exports the footer, which is not correct. So I followed a similar approach to DataViews and created (and exported) BulkActionToolbar which holds the selection checkbox, the message, and the actions. It doesn't contain the pagination and is more flexible for free composition.
2. Fixed a logic bug in useIsMultiselectPicker where we wouldn't check the existence of actions, which also could lead to wrong aria props.
3. Exported the footer. This component is the one that is going to be used by the revisions (other PR).

Testing Instructions

1. No visible changes anywhere. Everything should work as before
2. You can confirm in storybook (npm run storybook:dev) or existing picker usages like the media picker

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: ntsekouras <[email protected]>
Co-authored-by: Mamaduka <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

Size Change: +156 B (0%)

Total Size: 8.59 MB

📦 View Changed

Filename	Size	Change
build/modules/content-types/index.min.js	159 kB	+2 B (0%)
build/scripts/edit-site/index.min.js	297 kB	+11 B (0%)
build/scripts/editor/index.min.js	470 kB	+2 B (0%)
build/scripts/media-utils/index.min.js	114 kB	+141 B (+0.12%)

_{compressed-size-action}

[Mamaduka]

Thanks, @ntsekouras!

This tests well for me in Storybook, and refactoring makes sense.