http: add client certificate user auth middleware

This populates the authenticated user from the client certificate
common name.

Also added tests for the existing client certificate functionality.

lib/http/testdata/emptyclient.key

@@ -0,0 +1,28 @@
+-----BEGIN PRIVATE KEY-----
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+fOG8ouxBmrh03LCmnvYVGluA
+-----END PRIVATE KEY-----