Enable the ability to include default styles alongside Maui Controls

Currently there's not a reasonable way to package default styles inside Controls. If we try to add our own DefaultDictionary into Merged Dictionary it gets cleared out after InitalizeComponent is called.

At a later point I'll elaborate on the spec here but the ideal end goal here would be
- The ability to package XAML based themes with Controls
- The ability to define the concept of Defaults for controls stylings that feed from our themes.
- We recently added default styles into our [templates](https://github.com/dotnet/maui/blob/main/src/Templates/src/templates/maui-mobile/Resources/Styles.xaml#L7-L32). Ideally we'd be able to feed our control defaults off these colors. We would have the same idea as "AccentColor" etc.. that would act as defaults for colors that users could define to achieve consistent theming

https://github.com/dotnet/maui/pull/6255