Implement AbsoluteLink() on RequestHandler

[GuySartorelli]

It's very common to implement an AbsoluteLink() method on custom controllers - but really that should already be implemented, just link Link() is.

Acceptance Criteria

• RequestHandler provides an absolute link out-of-the-box.
• This is tested to make sure it works with subsite and fluent.
• Signature doesn't conflict with typical signature for AbsoluteLink method.

PR

• API Add AbsoluteLink method to RequestHandler #10749