added validations with kubebuilder's markers

[itamar-marom]

Fixes #25

Description

Kubebuilder supports object validation markers. Instead of validating the object inside the reconcile function (in the verifyDruidSpec function, we should let Kubebuilder do it.

For every required field we're adding the // +required marker.
For example, nodeType that now looks like this:

// Druid node type
// +required
// +kubebuilder:validation:Enum:=historical;overlord;middleManager;indexer;broker;coordinator;router
NodeType string `json:"nodeType"`

will return:

The Druid "tiny-cluster" is invalid: spec.nodes.brokers.nodeType: Required value If not specified.
The Druid "tiny-cluster" is invalid: spec.nodes.brokers.nodeType: Unsupported value: "brokerr": supported values: "historical", "overlord", "middleManager", "indexer", "broker", "coordinator", "router" If using unsupported value.

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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

[AdheipSingh]

Thank You @itamar-marom