[GOVCMSD11-566] Add new environment variables for SSO.

[mingsong-hu]

Add two new environment variables:

• SIMPLESAMLPHP_SP_ASSERTION_ENCRYPTION (defaults to false): Maps to assertion.encryption. When set to true, the SP will reject unencrypted assertions. Default as false for backwards compatibility.
• SIMPLESAMLPHP_SP_WANT_ASSERTIONS_SIGNED (defaults to false): Maps to WantAssertionsSigned. When set to true, the SP requires assertions to be explicitly signed. Default as false for backwards compatibility.