fix: set FieldPath for SequenceNode elements

[sdowell]

The FieldPath was not being set for nodes underneath a SequenceNode
during fieldspec's traversal. This is in part because handleSequence
uses VisitElements in contrast to a PathGetter as is done by handleMap.

The accuracy of FieldPath is more relevant now with the recently added
support of mutation trackers in filtersutil. This change fixes the
case where a mutation tracker callback is called on a SequenceNode
element and the node does not have an accurate FieldPath value.

For example:

apiVersion: v1
kind: Pod
metadata:
  name: app
spec:
  containers:
  - name: store
    image: redis:6.2.6

For the above input, the following FieldPath would be seen for the image node from the perspective of a mutationTracker:
Before this change: image
After this change: spec.containers.image

[k8s-ci-robot]

Hi @sdowell. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[natasha41575]

/ok-to-test

[natasha41575]

PTAL at the CI failure, other than that LGTM.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: natasha41575, sdowell

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[sdowell]

PTAL at the CI failure, other than that LGTM.

@natasha41575 Looks like I forgot to run gofmt before pushing my commit - should be good to go now

[natasha41575]

/lgtm