Refactor format option validation and tests

[AndrewSirenko]

Is this a bug fix or adding new feature?
bug fix

What is this PR about? / Why do we need it?
Split off parts of #1706 that refactor the format option parameter validation and controller unit tests.

What testing is done?
make test

[k8s-ci-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[github-actions]

Code Coverage Diff

File	Old Coverage	New Coverage	Delta
github.com/kubernetes-sigs/aws-ebs-csi-driver/pkg/driver/controller.go	85.1%	85.8%	0.7
github.com/kubernetes-sigs/aws-ebs-csi-driver/pkg/driver/node.go	80.3%	80.1%	-0.2
github.com/kubernetes-sigs/aws-ebs-csi-driver/pkg/util/util.go	85.7%	86.4%	0.6

[torredil]

/lgtm
/approve

[k8s-ci-robot]

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