fix(nil pointer): storage class reference

[itamar-marom]

Fixes #1

Description

This PR fixes the issue where the operator gets a nil pointer exception while trying to reach the storage class name of a storage class name template when it is not specified.

Important:
StorageClassName is generally not a required field (https://github.com/kubernetes/kubernetes/blob/0cabb55f7c05eb269ae6f7292271833f095657ec/pkg/apis/core/types.go#L461).
If a storage class is not provided, Kubernetes checks the default StorageClass (https://kubernetes.io/docs/concepts/storage/storage-classes/#default-storageclass).
This is tricky because there could be multiple storage classes and very dangerous to Druid (https://github.com/kubernetes/kubernetes/blob/0cabb55f7c05eb269ae6f7292271833f095657ec/test/e2e/framework/pv/pv.go#L822).
Therefore, we will require users to specify a storage class when they add a volume claim template to their StatefulSets.

Backward compatibility:
Not specifying storageClassName is a very unpopular behavior but it may happen. Such users will have to check their Druid workloads and add a storage class to their claim templates

This PR adds a feature of validation of storage class names inside a Node of kind StatefulSet. It also adds a nil check before trying to reach this field.

Tests are also added to check both logics.

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This PR has:

• been tested on a real K8S cluster to ensure creation of a brand new Druid cluster works.
• been tested for backward compatibility on a real K*S cluster by applying the changes introduced here on an existing Druid cluster. If there are any backward incompatible changes then they have been noted in the PR description.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added documentation for new or modified features or behaviors.

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Key changed/added files in this PR

• handler.go
• volume_expansion.go
• volume_expansion_test.go

[AdheipSingh]

@itamar-marom thanks for this.
@avtarOPS can you help in reviewing this one. Ill review this too

[avtarOPS]

LGTM

[AdheipSingh]

thanks @itamar-marom
Neat code and kudos for adding the test.

Just a nit pick, ideally i would want to get rid of all loggers and emit events instead.
Regardless will merge this.