Re-add wai-saml2

[ysangkok]

wai-saml2 was removed in #7431 before it got crypton-x509 compatible.

Now wai-saml2-0.6 is out, and it has the compatibility. Since the package was
removed by a curator, and not by the author, I am allowing myself to re-add the
package to the author's section, given that the package author originally added
it themselves in #5492.

Checklist:

• Meaningful commit message, eg add my-cool-package (please don't mention build-constraints.yml)
• At least 30 minutes have passed since uploading to Hackage
• If applicable, required system libraries are added to 02-apt-get-install.sh or 03-custom-install.sh
• (recommended) Package have been verified to work with the latest nightly snapshot, e.g by running the verify-package script
• (optional) Package is compatible with the latest version of all dependencies (Run cabal update && cabal outdated)