Fix missing requests when only limits are supplied

[dewjam]

1. Issue, if available:
#1573

2. Description of changes: (Edit):
There are two primary changes included in this PR.

First, we will now merge Pod resource Limits into Requests when Requests are not defined. Normally when a pod is created, if only Limits are supplied for a specific resource (no Requests), then the PodSpec is updated so the Requests = Limits when the pod event is created. However, when Karpenter calculates node overhead, it looks at the limits/requests in the PodSpec template of applicable DaemonSets. If only Limits are supplied in the PodSpec template, then requests are never updated to equal limits for resources where requests are not defined. The result is pods can be packed and scheduled onto an instance type which doesn't have enough resources and pods stay in a "Pending" state.

For example, if a manifest contains the below:

resources:
  limits:
    cpu: 1

The pod will be created as such:

resources:
  limits:
    cpu: 1
  requests:
    cpu: 1

Second, we will now consider InitContainer resources when scheduling/packing pods onto nodes. In short, here is the formula to calculate the resource "Ceiling" for a pod:

1. Get sum of all app container resources for the pod
2. Get max of all init container resources for the pod (we get a Max because InitContainers are started sequentially)
3. Get max of sum(appContainers) and max(initContainers)

3. How was this change tested?
Added a tests to validate the behavior after the change. Tested manually with a DaemonSet with only limits to ensure it was scheduled on a node which has enough resources. Tested manually to ensure init container resources are considered when scheduling.

4. Does this change impact docs?

• Yes, PR includes docs updates
• Yes, issue opened: link to issue
• No

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

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[bwagner5]

lgtm

[bwagner5]

is this test named wrong or is it supposed to be InitResourceRequirements rather than ResourceRequirements?