Validate assertion format

[peppelinux]

This PR adds some checks on assertion element.
The first commit is not so clean, due to the previous PR that makes some conflicts, please merge the PR previous to this to avoid messes.

checks implemented:

• assertion version if present MUST be '2.0'
• NameID Format, if present must be valid
• SubjectConfirmation Data, InResponseTo and notOnAfter

checks that needs unit test refactor (that's commeted in the code)

• IssueInstant
• NameQualifier

All Submissions:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
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• Have you written new tests for your changes?
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