chore: allow PIA adoption by clusterName, namespace, and ServiceAccount

[michaelhtm]

Issue #2684

Description of changes:
With these changes, we allow users to use ClusterName, Namespace, and
ServiceAccount to adopt a PodIdentityAssociation resource when using
adopt-or-create if AssociationID is not provided.

Note: this will not work when using adopt adoption policy

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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This pull-request has been approved by: michaelhtm

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@michaelhtm: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
eks-verify-code-gen	f116109	link	false	/test eks-verify-code-gen

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