Validate Destination

[peppelinux]

This PR introduces a validation on the Destination attribute value, in Response.
Destination value MUST be present if binding is HTTP-REDIRECT or HTTP-POST, and correctly valued

fixes: #770

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[c00kiemon5ter]

Does the specification define that?

What I see is that the Destination attribute is optional, verified by the schema

Destination [Optional]
A URI reference indicating the address to which this request has been sent. This is useful to prevent
malicious forwarding of requests to unintended recipients, a protection that is required by some
protocol bindings. If it is present, the actual recipient MUST check that the URI reference identifies
the location at which the message was received. If it does not, the request MUST be discarded.
Some protocol bindings may require the use of this attribute (see [SAMLBind]).

<attribute name="Destination" type="anyURI" use="optional"/>

[peppelinux]

@c00kiemon5ter Ok, I believe that this PR can be reopened following the reason that the Destination MAY be omitted, because it's optional, BUT if present it MUST be validated upon a match on valid ones.

This means that https://github.com/IdentityPython/pysaml2/pull/782/files#diff-93d65463e66cc55182c30d4629747c4db87f8230056f81497719a0144f44749cR415 would be removed and the check will be done only "if destination" is True