Request Signatures
If signature_key is defined, proxied requests will be signed with the
GAP-Signature header, which is a Hash-based Message Authentication Code
(HMAC)
of selected request information and the request body see SIGNATURE_HEADERS
in oauthproxy.go.
signature_key must be of the form algorithm:secretkey, (ie: signature_key = "sha1:secret0")
For more information about HMAC request signature validation, read the following: