Allow External Lookup Fields to utilize a "Select All" button

[#1]

Description - When using a $$HOST-based external lookup field and maintaining a data table, such as a list of US States, users currently have to click on each valid option that appears in the picklist for a multi-select-allowed External lookup field. 

Why is this feature important to you - This will bring parity to the External Lookup Field with the Multi-select Dropdown field. Much of the discussion around external lookup fields is that they can be used as a dependency-driven list of results that helps control front-end user input, especially when flowing through a custom form for an intake process. This really supports staying away from the "Garbage in, garbage out" trap of users mis-selecting options in an intake process.

How would you like the feature to work - When "Multi Select Dropdown" is selected in the field setup, users should be given a UI-button that responds the same way the current Multi-select options (Dropdown, Checkboxes) works. A "Select All" option should populate all of the choices that are valid for the JSON path, and then from there, users should be able to use the "x" button on the individual choices they DO NOT want to include. When the 1st option removed, the "Select All" button should also clear, allowing for it to be re-selected, and any options that were removed, should be re-added to the list within the field.

Current Behaviour - Users have to click every valid option in an external lookup field result. 

Bonus fix/Observation!!! - The user experience when selecting multiple options from an external lookup seems to be wildly inconsistent. Some users can click a result, and the list of possible results stays open allowing them to quickly click more options to add to the selection. Other users have the external lookup dropdown interface close with each and every selection, requiring an additional click to re-open the selection field, and another click to select an item, which subsequently closes the field again.