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

.gitignore

@@ -1,4 +1,5 @@
 /.inventory-cache
+/.lookup-cache
 /.vault-secret.gpg
 .fact-cache
 /secure.yaml