feat(al2/bootstrap): add dedicated flag for max pods

[mselim00]

Issue #, if available:

Description of changes: Creates a new CLI flag for setting max pods. This is mainly to handle the case where a user wants to customize max pods independent of what we would set (including if they want to use the existing max-pods-calculator.sh) and ensure that the system reserved logic gets updated accordingly.

Memory reserved is calculated as a linear function of max pods (11 * maxPods + 255). If max pods is passed in kubelet-extra-args, we still calculate memory reserved based on the default maxPods value -- either retrieved from eni-max-pods.txt, or computed from equivalently computed from DescribeInstanceType call if that's not set. It's messy to try to parse the flag out of that list of flags, so this instead adds a flag to better capture the functionality, and also makes it opt-in to avoid a shift in functionality for users who might have indirectly come to rely on the omission.

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