Add configuration to limit the max number of container processes

## Motivation

- See [https://lablup.atlassian.net/browse/BA-6508](https://lablup.atlassian.net/browse/BA-6508).

## Required Features

- Add the following **per-resource-group** configuration for super-admins:
- A flag to enable or disable the pid limits. (default: on)
- The maximum number of processes. (default: 4096)

## Impact

- Users can no longer spawn indefinite number of processes, which may degrade the system performance and affect the workload sharing the same host.

## Testing Scenarios

- Let’s add unit tests for agents that sets the pid limit as a small number and executes a script within a kernel container that spawns many child processes above the limit.

JIRA Issue: BA-6521

Start by reading the agent unit tests and the code that launches kernel containers; the issue does not name specific files or entry points. Trace how per-resource-group settings reach agents, then test a small process limit with a script that spawns children. Done means super-admins can toggle the limit and set its maximum, with defaults of on and 4096, and the agent enforces it.

Written by the indexing model from the issue text.