Use an interface to construct Populator Pod Spec

[nahwneeth]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test

/kind feature

/kind flake

What this PR does / why we need it:
Adds an interface to provide the library user the ability to construct a pod spec tailored for their workload. In issue #170 I've tried to described how this can be used to provide trusted CA certificates to containers using cluster trust bundles irrespective of the variant of linux container used.

Which issue(s) this PR fixes:

Fixes #170

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Library users can provided their own Populator Pod Spec.
The existing PodConfig is still valid and is the default implementation.

[k8s-ci-robot]

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