Make polling `apiclient update check` frequency configurable

### 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

```yaml
- 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.