Add DataTemplate extensions to allow typesafe binding just like BindableObject

[scastria]

Description of Change

Allow typesafe binding on DataTemplate just like we have for BindableObject. This allows better support for refactoring such that when a bound property name changes, it will be found in the lambda expressions and be updated too.

My first attempt added these extension methods to DataTemplateExtensions.cs. Then I noticed that there was already an extension method in TemplateExtensions.cs. Therefore, I moved my changes to TemplateExtensions.cs to better match what was already there.

API Changes

Added:

• DataTemplate.SetBinding(this DataTemplate self, BindableProperty targetProperty, string path, BindingMode mode = BindingMode.Default, IValueConverter converter = null, string stringFormat = null);
• DataTemplate.SetBinding(this DataTemplate self, BindableProperty targetProperty, Expression<Func<TSource, object>> sourceProperty, BindingMode mode = BindingMode.Default, IValueConverter converter = null, string stringFormat = null);

PR Checklist

• [ X] Has tests (if omitted, state reason in description)
• [ X] Rebased on top of master at time of PR
• Changes adhere to coding standard
• Consolidate commits as makes sense

[jassmith]

After internal debate we have opted not to include this due to its usage of Expression which expands out quite poorly in iOS AOT land resulting in bloating of binary sizes.

[scastria]

Can you elaborate a bit on why BindableObject's use of the Expression does not result in bloated binaries but this change would?

I suppose I could add these extension methods in all of my apps to avoid using simple binding strings, but I was hoping to get this added to Forms so everybody can avoid simple strings which are not refactor safe.