Add Pinterest to the share providers

There’s no UI or anything to configure share providers, but if you’re a developer, you can use some code like this:

add_filter(
	'web_stories_share_providers',
	static function ( $share_providers ) {
		$share_providers[] = array(
			'provider'         => 'pinterest',
			'data-param-media' => get_the_post_thumbnail_url( get_post(), 'web-stories-poster-portrait' ),
		);

		return $share_providers;
	}
);

The list of possible providers can be found at https://amp.dev/documentation/components/amp-social-share/#pre-configured-providers

thanks!