Please add Combo Box to .Net Maui

[HimanshuBansal112]

Description

I would like to have a combo box which has combined text box and picker. While some might say auto complete can also work for it which was asked for, in #13545, auto complete does not have list to choose from, it only shows from what user has typed first, that is when "a" is typed, it will show list for that and might even work for in between. A combo box can be useful when user first checks list, then choose/modify it according to their need.

Public API Changes

<ComboBox Header="Colors" PlaceholderText="Pick a color" Width="200">
    <x:String>Blue</x:String>
    <x:String>Green</x:String>
    <x:String>Red</x:String>
    <x:String>Yellow</x:String>
</ComboBox>

Intended Use-Case

In billing program I made, we have some items that are more common, a user might check that the list include the item customer has asked, if it is not present, the user will not have to go to other page to be just able to add that item, instead they can keep making bill, and add those items later. This have several use cases like this in other programs I have made. Due to it not being available, I had to make it in windows forms app, though I would like to have this feature in net maui so that some cross platform apps needing this can use this. Here I needed this for better ui look and for mac os support as windows forms app does not have this.