Follow-up to #26506

The endpoint that provides the storage used is registered by Jetpack plugin, meaning that is not accessible from VideoPress standalone plugin: Let;s figure out how to fix it. Probably registering a new endpoint in the VideoPress env.

Fetch local videos, using same API that we use for videopress videos.

Allow user to upload a video from local library. Already supported by https://github.com/Automattic/jetpack/blob/trunk/projects/packages/videopress/src/class-uploader-rest-endpoints.php

Allow user to upload a poster image from library. Already supported by https://github.com/Automattic/jetpack/blob/trunk/projects/packages/videopress/src/class-wpcom-rest-api-v2-endpoint-videopress.php#L101

While uploading, we should bring a dialog for user it tries to leave the page, since this will breaks the upload.

When uploading, we should not store the uploaded video together with loaded videos. This will allow us to avoid blocking the screen while uploading the video, and the user could navigate across the places while uploading.

When we navigate to edit details page and try to update the poster by selecting a frame, we should have a placeholder loading to avoid jumps in the screen.