ComboBox: Lazy loading implementation

[niyap]

After a discussion within the team, we decided to create a backlog item, in order to find the best way for "lazy loading" implementation in ui5-combobox component.

We have to investigate whether we should introduce an event similar to "loadItems" in sap.m.ComboBox - "This event is fired which is fired when the end user clicks the combo box button to open the dropdown list and the data used to display items is not already loaded. Alternatively, it is fired after the user moves the cursor to the combo box text field and perform an action that requires data to be loaded. For example, pressing F4 to open the dropdown list or typing something in the text field fires the event."
If such an event is not a proper solution, we have to find a better way to cover all supported scenarios from the component - for example opening the picker via F4/ Alt + Down/ Dropdown button or navigating through the items via arrow UP/DOWN in the input field, while the picker is closed and so on.

[niyap]

Related issue: #3759

[hristop]

Internal BLI created: FIORITECHP1-32718