Implement POST-as-GET requests (#6522)

* Setup an integration tests env against Pebble, that enforce post-as-get

* Implement POST-as-GET requests, with fallback to GET.

* Fix unit tests

* Fix coverage.

* Fix or ignore lint errors

* Corrections after review

* Correct test

* Try a simple delegate approach

* Add a test

* Simplify test mocking

* Clean comment

tests/certbot-pebble-integration.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+# Simple integration test. Make sure to activate virtualenv beforehand
+# (source venv/bin/activate) and that you are running Pebble test
+# instance (see ./pebble-fetch.sh).
+
+cleanup_and_exit() {
+    EXIT_STATUS=$?
+    unset SERVER
+    exit $EXIT_STATUS
+}
+
+trap cleanup_and_exit EXIT
+
+export SERVER=https://localhost:14000/dir
+
+./tests/certbot-boulder-integration.sh