Publicize: allow to filter the publicized URL

Doing this will allow to filter the URL and instead of sending the original permalink, users could send a URL shortened through core's wp.me or third party services as bit.ly.

This is possible in VIP sites but not currently in third party Jetpack sites:
https://vip.wordpress.com/documentation/customizing-publicize-sharing-behavior/#implement-your-own-url-shortening-service

Requested in 842186-zen

Start with the linked VIP documentation on customizing Publicize sharing behavior and compare it with the third-party Jetpack Publicize flow. Identify the Publicize entry point that assembles the shared URL and verify that the finished change lets sites replace the original permalink with a wp.me or third-party shortened URL.

Written by the indexing model from the issue text.