Replace coalescer implementation with new coalescer package

[ConnorJC3]

Is this a bug fix or adding new feature?

New feature

What is this PR about? / Why do we need it?

Replaces the previous mixed-in coalescer implementation with the new package from #2024

What this PR does NOT do, on purpose:

• Significantly change the unit tests: The unit tests could be refactored to mock the coalescer instead of cloud for the ModifyVolume tests (and probably should be, in the future). I chose not to do this in this iteration because it would be a significant overhaul, and keeping the unit tests as close as possible to the original best demonstrates this PR introduces no new bugs. I plan to refactor the coalescing-related tests in a follow up PR.
• Refactor cloud.ModifyDiskOptions and cloud.ModifyOrResizeDisk: I think the remaining ModifyVolume code could be greatly simplified by moving the size into ModifyDiskOptions (rather than having it as a separate parameter), but this PR is not the place to do so

What testing is done?
CI/Unit Tests

[github-actions]

Code Coverage Diff

File	Old Coverage	New Coverage	Delta
github.com/kubernetes-sigs/aws-ebs-csi-driver/pkg/driver/controller.go	83.0%	82.8%	-0.2
github.com/kubernetes-sigs/aws-ebs-csi-driver/pkg/driver/controller_modify_volume.go	85.4%	77.2%	-8.2

[torredil]

−316 Thank you for this. This change significantly improves the modify volume implementation.

/lgtm

[AndrewSirenko]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AndrewSirenko

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [AndrewSirenko]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[torredil]

/retest

[torredil]

/retest