Display WordPress Posts Widget: Add the "Read More" option for each post

### Request

* It's great to have an option for displaying the `Read More` link for each remote post content.
* It's even better if we can have a filter to modify the display of each remote post content.

This is the reason.

```
I find missing is the option to add a 'read more' link, at the end of an excerpt from the remote site article.

I've noticed that elderly people - even if they're modern enough to have a smartphone - don't get that they can click / tick the featured image to go to the original article.
```

### What I found out

I found out this filter

* https://developer.jetpack.com/hooks/jetpack_display_posts_widget_content/
* https://github.com/Automattic/jetpack/blob/5.4/modules/widgets/wordpress-post-widget.php/#L835

However, this filter is applied to the whole widget content, it's not possible to apply for each post in this widget content.

### Other notes:

* Ticket: 740159-zen.

Start with modules/widgets/wordpress-post-widget.php around line 835 and the jetpack_display_posts_widget_content filter linked in the issue. Trace how each remote post is rendered; done means each post can show a Read More link and its individual content can be modified through a filter, rather than only filtering the whole widget.

Written by the indexing model from the issue text.