Add support for XML documentation in the generator

[rolfbjarne]

Ref: #14372, step 4

The idea is to add support in the generator for writing XML documentation based on an attribute in the api definition:

This would be the attribute:

public class DocumentationAttribute : Attribute {
    public DocumentationAttribute ()
    {
    }

    public DocumentationAttribute (string xml)
    {
        this.Xml = xml;
    }
 
    public string Xml { get; set; }
    public string Include { get; set; }
    public string XmlPath { get; set; }
}

and this would be how to use it:

[Documentation (
@"
/// <summary>
/// Documentation for UIView
/// </summary>
")
]
public interface UIView {
}

[Documentation (Include = "/path/to/file.xml", XmlPath = "Type@[FullName='UIKit.UIView']/Docs/*")]
public interface UIView {
}

MAUI example with include path: https://github.com/dotnet/maui/blob/6377c3259387913f3acc7f7858028f7c97a0480b/src/Controls/src/Core/Application.cs#L17

[rolfbjarne]

It seems we don't need an additional attribute, we can just write normal xml docs:

/// <summary>Summar of SomeType</summary>
[BaseType (typeof (NSObject)]
interface SomeType {}

it's possible to load the xml doc in the generator and forward it to the generated code.