Add destination field to AuthnRequest

[Philonous]

Summary AuthnRequest has an optional attribute, Destination, which is however required for signed requests. This PR adds this attribute.

Checklist

• [x] All definitions are documented with Haddock-style comments.
• [x] All exported definitions have @since annotations.
• [x] Code is formatted in line with the existing code.
• [x] The changelog has been updated.

[Philonous]

Ah, thanks for taking care of this!