plugins: Add lookup cache plugin

One major weakness with Ansible's "lookup" plugins is that they are
evaluated _every single time they are used_, even indirectly.  This
means, for example, a shell command could be run many times, potentially
resulting in different values, or executing a complex calculation that
always provides the same result.  Ansible does not have a built-in way
to cache the result of a `lookup` or `query` call, so I created this
one.  It's inspired by [ansible-cached-lookup][0], which didn't actually
work and is apparently unmaintained.  Instead of using a hard-coded
file-based caching system, however, my plugin uses Ansible's
configuration and plugin infrastructure to store values with any
available cache plugin.

Although looking up the _pyrocufflink.net_ wildcard certificate with the
Kubernetes API isn't particularly expensive by itself right now, I can
envision several other uses that may be.  Having this plugin available
could speed up future playbooks.

[0]: https://pypi.org/project/ansible-cached-lookup

group_vars/wildcard-cert.yml

@@ -1,21 +1,17 @@
-apache_ssl_certificate_data: >-
-  {{
-  query(
+pyrocufflink_wildcard_cert_secret: >-
+  {{ lookup(
+    "cache",
     "kubernetes.core.k8s",
     kind="Secret",
     namespace="default",
     resource_name="pyrocufflink-cert"
-  )[0].data["tls.crt"]
-  | b64decode
-  }}
+  ) }}
 
-apache_ssl_certificate_key_data: >-
-  {{
-  query(
-    "kubernetes.core.k8s",
-    kind="Secret",
-    namespace="default",
-    resource_name="pyrocufflink-cert"
-  )[0].data["tls.key"]
-  | b64decode
-  }}
+pyrocufflink_wildcard_cert: >-
+  {{ pyrocufflink_wildcard_cert_secret.data["tls.crt"] | b64decode }}
+
+pyrocufflink_wildcard_key: >-
+  {{ pyrocufflink_wildcard_cert_secret.data["tls.key"] | b64decode }}
+
+apache_ssl_certificate_data: "{{ pyrocufflink_wildcard_cert }}"
+apache_ssl_certificate_key_data: "{{ pyrocufflink_wildcard_key }}"