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

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

[mbg]

Looks good to me! Thank you for adding this ✨

There's just comment about the versioning. Let me know if you want to act on that - otherwise, I am happy to merge this as is and then deal with the versioning issue myself later.

[Philonous]

Ah, thanks for taking care of this!