[combo-box] Visually communicate if ComboBox allows custom values

[juuso-vaadin]

Describe your motivation

Currently ComboBox doesn't communicate to the user if the input accepts custom values or not.
Blurring the field will clear the input if custom values are not allowed.

Describe the solution you'd like

Overlay could be used to communicate if no items are matching the input.

In addition if custom values are allowed, the overlay could indicate that it's a new value that isn't in the list yet. The custom values may be added to the list of options or used as ad-hoc values and maybe the visuals could communicate that difference as well.

Describe alternatives you've considered

Using prefix or helper would affect the sizing which is not good. Suffix would be the least distracting, but still affects the area that remains for the value.

Additional context

No response