Add 50Mi memory requests for aws-node pod; update chart README

[jdn5126]

What type of PR is this?
enhancement

Which issue does this PR fix:
N/A

What does this PR do / Why do we need it:
This PR adds a 50Mi memory request field to the aws-node pod. Note that this request applies to the init container, main container, and network policy agent container by default.

This change is made for two reasons:

1. It is good practice to specify CPU and memory requests
2. This help node autoscalers like Karpenter avoid selecting instance sizes that are too small for pods to run on.

This PR also updates the README in the aws-vpc-cni helm chart to provide instructions on how to set the field manager to helm when installing the chart. Since Kubernetes recommends using server-side apply, the README uses it to apply the rendered helm chart.

If an issue # is not available please add repro steps and logs from IPAMD/CNI showing the issue:

Testing done on this change:
Manually verified that the requests apply

Will this PR introduce any new dependencies?:
No

Will this break upgrades or downgrades? Has updating a running cluster been tested?:
No, Yes

Does this change require updates to the CNI daemonset config files to work?:
Yes

Does this PR introduce any user-facing change?:
Yes

Specify 50Mi memory request for containers in the aws-vpc-cni helm chart.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.