Make polling `apiclient update check` frequency configurable

[jpmcb]

Frequency of polled updates

Currently, polling for a new update seem to be every 10 seconds:

https://github.com/bottlerocket-os/bottlerocket-update-operator/blob/c2e9504d7671c263ecdf7ea59fea69609b92e15e/agent/src/apiclient.rs#L123-L128

It would be great if there was an brupop setting that made polling for updates configurable

        - name: POLLING_UPDATE_FREQUENCY
          value: 10

where the value is an expected length of seconds

Why?

Some users may not want egress traffic to the updates endpoint very frequently. Or maybe they want to poll for an update more frequently. This may be a network traffic restriction, the fact that nodes are deployed on edge (and polling for frequent updates are not ideal), etc.