Support download options for web books

### Problem

Related: #6585

### A clear and concise description of what you want to happen
Visiting the book page (e.g. http://localhost:8080/books/OL20431791M/Flatland_A_Romance_of_Many_Dimensions) should show download options for web books, to the extent they are available.

### Expected behaviour / screenshots (ex: Figma design screenshots for UI feature)
Visiting a book page with a web book shows, in the left-hand menu, download options appropriate for the web book.

### Additional Context
#6585 adds web books, but the `DirectProvider` class does not support download options.

### Proposal & Constraints

### Which suggestions or requirements should be considered for how feature needs to appear or be implemented?
We need to think about how this would work, and the extent to which consideration of direct book providers needs to be taken into account.

### Leads

### Related files

The former `openlibrary/templates/book_providers/direct_download_options.html` may provide some useful context when implementing this:
```
$def with(url)

### Stakeholders

@cdrini

### Instructions for Contributors

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Start with the book page at /books/OL20431791M/Flatland_A_Romance_of_Many_Dimensions, the DirectProvider class, and the former openlibrary/templates/book_providers/direct_download_options.html template. Determine how available download options for web books and direct book providers should appear in the left-hand menu; done means an eligible web book displays appropriate options.

Written by the indexing model from the issue text.