ACME: use Cryptography (if a new enough version is available) instead of OpenSSL (#42170)

* Collecting PEM -> DER conversions.

* Using cryptography instead of OpenSSL binary in some situations.

* Moving key-to-disk writing for key content to parse_account_key.

* Rename parse_account_key -> parse_key.

* Move OpenSSL specific code for key parsing and request signing into global functions.

* Also using cryptography for key parsing and request signing.

* Remove assert statements.

* Fixing handling of key contents for cryptography code path.

* Allow to disable the use of cryptography.

* Updating documentation.

* 1.5 seems to work as well (earlier versions don't have EC sign function). Making Python 2.x adjustments.

* Changing option to select_crypto_backend.

* Python 2.6 compatibility.

* Trying to test both backends separately for acme_account.

* Also testing both backends separately for acme_certificate and acme_certificate_revoke.

* Adding changelog entry which informs about select_crypto_backend option in case autodetect fails.

* Fixing YAML.

changelogs/fragments/42170-acme-backend-selection.yaml

@@ -0,0 +1,8 @@
+---
+minor_changes:
+- "The acme_account and acme_certificate modules now support two backends:
+   the Python cryptograpy module or the OpenSSL binary. By default, the
+   modules detect if a new enough cryptography module is available and
+   use it, with the OpenSSL binary being a fallback. If the detection
+   fails for some reason, the OpenSSL binary backend can be explicitly
+   selected by setting select_crypto_backend to openssl."