Sharing: add option to use sharing_display() outside of the loop

It would be nice if it were possible to use `sharing_display()` outside of the loop and specify a post ID that would be used in the buttons.

Suggested here:
https://wordpress.org/support/topic/advanced-custom-fields-and-jetpack-sharing?replies=1&view=all

Start by locating the existing sharing_display() implementation and examining how it obtains the current post inside the loop. Determine how an explicit post ID should be accepted and used for the sharing buttons, then verify that the buttons render correctly when called outside the loop. The linked WordPress support discussion provides the original use case.

Written by the indexing model from the issue text.