Pinterest sharing button fails to retrieve an image

#### Steps to reproduce the issue

1. Go to https://thepostmansknock.com/incredible-bullet-journal-ideas-examples/
2. Click on the AMP Pinterest sharing button under the title and excerpt.
4. See that the Pinterest pop-up window does not pull up an image to share.

#### What I expected

I expected an image to be pulled up to share.

#### What happened instead

No image is displayed for the user to pin.

The Pinterest AMP button needs a `data-param-media` attribute to pull up an image (see https://amp.dev/documentation/components/amp-social-share/#pre-configured-providers). I think the plugin should offer a filter in the function `render_sharing_html` in `jetpack/3rd-party/class.jetpack-amp-support.php` for users to be able to add more parameters to the social share buttons.

Start in jetpack/3rd-party/class.jetpack-amp-support.php at render_sharing_html and review how the AMP Pinterest sharing button is assembled, using the AMP social-share provider documentation as the reference. Done means the button can include the media parameter needed for Pinterest to display an image, while preserving existing sharing behavior.

Written by the indexing model from the issue text.