Add "buffer memory" reservation in the agent

Currently we have an agent.toml config: `reserved-memory = "1g"` to exclude the memory size used by the OS and agent from the user allocations.

@712020:489db721-7ccc-45fa-9a44-cb983a699ca3 has suggested to add another type of reserved memory, called `buffer-memory`, representing various kernel-level I/O allocations and buffer cache.

The desired effect is to reduce the chance of OOM errors and improve explainability to the customers/admins.

refs) [https://kubernetes.io/docs/tasks/administer-cluster/reserve-compute-resources/](https://kubernetes.io/docs/tasks/administer-cluster/reserve-compute-resources/)

JIRA Issue: BA-2246