Move ocsp.py to public api (#7744)

We should move ocsp.py to public API, as an upcoming OCSP prefetching functionality in Apache plugin relies on it, and as the plugins are note released in lockstep with the Certbot core, we need to be careful when changing those APIs.

* Move ocsp.py to public api

* Fix type annotations, move to pointing to an interface and fix linting

* Add certbot.ocsp to documentation table of contents

* Modify tests to reflect the changes in ocsp.py

* Add changelog entry

* Fix notAfter mock for tests

certbot/docs/api/certbot.ocsp.rst

@@ -0,0 +1,7 @@
+certbot.ocsp package
+======================
+
+.. automodule:: certbot.ocsp
+    :members:
+    :undoc-members:
+    :show-inheritance:

certbot/docs/api/certbot.rst

@@ -26,6 +26,7 @@ Submodules
    certbot.errors
    certbot.interfaces
    certbot.main
+   certbot.ocsp
    certbot.reverter
    certbot.util