fix(sanity): allow custom snapshot IDs

[jfbus]

What type of PR is this?

/kind bug

What this PR does / why we need it:

The sanity tests did not allow use of custom snapshot ids.

The "missing snapshot" ID used in tests would be considered invalid by most platforms.

This PR adds new snapshot ID generation methods to the IDGenerator interface, allowing the user to generate custom valid/invalid snapshot IDs.

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Add custom Snapshot ID methods to the IDGenerator interface. [action required] If you set a custom ID generator, you will need to implement the new methods. 

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: jfbus
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[k8s-ci-robot]

Hi @jfbus. Thanks for your PR.

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