providers/SAML: encryption support

[BeryJu]

Details

Add the option to encrypt the SAML assertion with a keypair

Also adds the option to sign either assertion or response or both

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Checklist

• [x] Local tests pass (ak test authentik/)
• [x] The code has been formatted (make lint-fix)

If an API change has been made

• [ ] The API schema has been updated (make gen-build)

If changes to the frontend have been made

• [ ] The code has been formatted (make web)

If applicable

• [ ] The documentation has been updated
• [ ] The documentation has been formatted (make website)