Publicize: Add ability to choose the Twitter card type

Currently, Jetpack will [choose the Twitter card](https://github.com/Automattic/jetpack/blob/3c4ae1a92429f18fda7b03515adceee17b480b35/class.jetpack-twitter-cards.php) that seems most appropriate when a post is shared to Twitter.

In some cases, a person may prefer to choose the Twitter card type. Is this a possible feature that could be added to Publicize?

Start with class.jetpack-twitter-cards.php, linked in the issue, and trace how Publicize currently selects a Twitter card type. Clarify which card types should be user-selectable and where that choice belongs in the Publicize flow; done means a person can choose the type instead of relying only on automatic selection.

Written by the indexing model from the issue text.