refactor(api): changing rollingUpdate default to true

[itamar-marom]

Fixes #81

Description

Druid requires a rolling update in order to apply changes with no downtime. The operator supports it with the rollingUpdate field.
Currently, this behavior is disabled by default for no reason.
This PR changes Druid's API rollingUpdate field to be true by default. This is done by using a Kubebuilder marker.

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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

• apis/druid/v1alpha1/druid_types.go

[itamar-marom]

This might be considered as a breaking change since users using the default rollingUpdate will now need to set it.