Anchor: Import podcast cover image to media library

#### Is your feature request related to a problem? Please describe.
We had included the podcast cover image in the Anchor block, but removed it to center the episode post design. However, the image is still included in the RSS feed data we populate the post with, if available, and podcasters may want to add it in.
See https://github.com/Automattic/jetpack/pull/18734#pullrequestreview-585088996 and https://github.com/Automattic/jetpack/pull/18734#issuecomment-775275542

Additionally, the image used in the podcast player block is hot-linked, rather than served from the WordPress site.

#### Describe the solution you'd like

We'd like to make the cover image available in the media library so it can be added to the podcast content without hot-linking.

Start by tracing the Anchor block, its RSS feed data, and the podcast player block to find where the available cover image is handled. Check how WordPress media-library imports are performed and verify the cover image is saved locally and can be added to the podcast content without hot-linking.

Written by the indexing model from the issue text.