Add a filter to edit the calendar single view card

[Valentina R]

This filter woocommerce_appointments_calendar_view_single_card adds content outside of the card. Many clients have been requesting the option to add data to the cards. Having a filter that allows adding data to the card would be very helpful.

Requested here.

[Željan Topić]

They can rebuild the whole card HTML and add their own data inside. This won't be edited since it will switch to Gutenberg cards in future.