proxy: add JWT request signing support (#19)

- Refactored middleware and request hander logging.
- Request refactored to use context.Context.
- Add helper (based on Alice) to allow middleware chaining.
- Add helper scripts to generate elliptic curve self-signed certificate that can be used to sign JWT.
- Changed LetsEncrypt scripts to use acme instead of certbot.
- Add script to have LetsEncrypt sign an RSA based certificate.
- Add documentation to explain how to verify headers.
- Refactored internal/cryptutil signer's code to expect a valid EC priv key.
- Changed JWT expiries to use default leeway period.
- Update docs and add screenshots.
- Replaced logging handler logic to use context.Context.
- Removed specific XML error handling.
- Refactored handler function signatures to prefer standard go idioms.

scripts/generate_self_signed_signing_key.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+# See: https://cloud.google.com/iot/docs/how-tos/credentials/keys#generating_an_es256_key_with_a_self-signed_x509_certificate
+# To generate an ES256 key with a self-signed X.509 certificate that expires far in the future, run the following commands:
+
+openssl ecparam \
+	-genkey \
+	-name prime256v1 \
+	-noout \
+	-out ec_private.pem
+
+openssl req \
+	-x509 \
+	-new \
+	-key ec_private.pem \
+	-days 1000000 \
+	-out ec_public.pem \
+	-subj "/CN=unused"