* Add Kubernetes Access support to Machine ID
This adds support for Kubernetes Access to Machine ID. Users can now
request access to a Kubernetes cluster with the new
`kubernetes_cluster` config field. When a request is configured, a
`kubeconfig.yaml` is generated which can be used to access the given
cluster.
As part of this, we also refactor a few parts of the bot code to help
cache various requests that config templates make frequently by
passing a new trivial `bot.B` interface to the `Render()` function.
For good measure, we also fix the `destination.Destination` stutter
by moving the interface to `bot.Destination`.
* Remove unused CLI param and fix docstring typo
* Cache cert authority requests
We now cache most cert authority requests from config templates and
purge the cache after each cert renewal.
* Move bot.B to config.Bot
* Add a matching marshaller for the KubernetesCluster type.
Noah Stride