[MultiInput] Add support for loading state

Is your feature request related to a problem?

The MultiInput component does not have a loading state and the user is required to implement one.

Describe the solution you'd like

While working on an issue relating to the content of the MultiInput not showing when fetching data, I realised that the component does not provide a native loading state. The effect of this is that the consumer of the component is required to implement a loading indicator of some sort and potentially leads to introducing custom styling outside of the Ui5 library.
I propose that this loading state should be natively supported by the MultiInput out-of-the box via a prop or a boolean value.

Describe alternatives you've considered

I tried using the <BusyIndicator /> to manually implement a loading state

Additional Context

No response

Organization

SAP

Declaration

• I’m not disclosing any internal or sensitive information.

Start by locating the MultiInput component entry point and reviewing how its current content and BusyIndicator-related behavior are implemented. Define the loading API and its expected visual behavior, then add the relevant component coverage so the work is complete when MultiInput natively exposes and reflects a loading state.