[Enhancement] Implement UpdateSourcetrigger

### Description

The `UpdateSourcetrigger` of `Binding` is extremely useful, please implement it in MAUI as in WPF.

### Public API Changes

Adding a new binding attribute - I don't see it would be a breaking change.

### Intended Use-Case

The `UpdateSourcetrigger` of `Binding` is extremely useful, please implement it in MAUI as in WPF.

I find it especially useful when validation is needed on user entered data (like when a number is being entered) before allowing the data to update the binding source.

And of course it would make migration from WPF or WinUI a bit easier (you can always code around this lack, but it's tedious to do so).

Start by reviewing MAUI's existing Binding API and the corresponding WPF UpdateSourceTrigger behavior. Done means adding the binding attribute and supporting validation-driven control of when user-entered data updates the binding source, while preserving the migration use case described in the issue.

Written by the indexing model from the issue text.