[BE] Add multi-select field support in the Experience manager visual editor

• As an editor, while creating or editing a document/page in experience manager, I want to be able to create/edit all fields the document/page directly in the experience manager without going to content perspective

Support multi select field in experience manager

Designs: https://www.figma.com/file/Ozr2KDZWQyxDUXxSdOZo9P/ContentPaas?type=design&node-id=1%3A18&mode=design&t=i5kIqE3QRm97USsp-1
 
 

Separate ticket for SaaS to be created

• this field can be only added through Selections plugin in Essentials- not available in Doc type editor.
• aim is not supporting it in doc type editor but only showing it in Experience manager