CSPL-2345 Upgrade Strategy for Single Site IDXC

[tgarg-splunk]

Implemented the design for the single site IDXC upgrade strategy for the Splunk Operator Level II support. Specifically:

• Added isIndexerClusterReadyforUpgrade to check if SHC is ready before a version upgrade of IDXC takes place.
• Added and fixed unit tests

[gaurav-splunk]

Closing this since it will be covered as part of https://github.com/splunk/splunk-operator/pull/1216