Add external auth handler

This commit adds an external auth handler which can be used to
wrap existing handlers, so that they delegate their requests
to an upstream URL before allowing a request to pass through
to an upstream API.

New handler tested with unit tests.

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

gateway/handlers/external_auth.go

@@ -0,0 +1 @@
+package handlers