utility function for getting user email from x509 certificate

tests/fixtures/artgarfunkel@uso.mil.crt

@@ -0,0 +1,29 @@
+-----BEGIN CERTIFICATE-----
+MIIE8DCCAtigAwIBAgIJALTstfJQuulmMA0GCSqGSIb3DQEBCwUAMCUxIzAhBgNV
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+wM+BoGmfKP7uq0GBfu/HengJEGk=
+-----END CERTIFICATE-----