fix multinamespace cache issue

[AdheipSingh]

Fixes #XXXX.

@itamar-marom can you help in reviewing this one.

Description

adheipsingh@Adheips-Air ~ % k logs -f cluster-druid-operator-6c7b966548-rj58x  -n druid-operator-system -c manager
2024-02-09T15:24:02Z	ERROR	setup	unable to start manager	{"error": "must specify more than one namespace to use multi-namespace cache"}
main.main
	/workspace/main.go:93
runtime.main
	/usr/local/go/src/runtime/proc.go:250

• This issue arises when we have both Namespace and NewCache enabled for a single namespace to watch.
• This PR add a check in watchNamespaceCache() to built cache only if multiple namespace are found, so runtime can default to watch namespace for single and NO namespace specified to watch.

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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

• MyFoo
• OurBar
• TheirBaz