Add pageToken response check to ListSnapshots gRPC API

[pwschuurman]

What type of PR is this?
/kind bug

What this PR does / why we need it:

Fixes memory leak in #2003, by breaking the loop when paginated results are complete.

Which issue(s) this PR fixes:

Fixes #2003

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Fix infinite loop in ListSnapshots that can cause memory usage to grow unbounded

[k8s-ci-robot]

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

Test name	Commit	Details	Required	Rerun command
pull-gcp-compute-persistent-disk-csi-driver-kubernetes-integration	e38bfeb	link	true	/test pull-gcp-compute-persistent-disk-csi-driver-kubernetes-integration
pull-gcp-compute-persistent-disk-csi-driver-e2e-windows-2022	e38bfeb	link	false	/test pull-gcp-compute-persistent-disk-csi-driver-e2e-windows-2022
pull-gcp-compute-persistent-disk-csi-driver-e2e-windows-2019	e38bfeb	link	false	/test pull-gcp-compute-persistent-disk-csi-driver-e2e-windows-2019

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[mattcary]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mattcary, pwschuurman

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