CSPL-2344 Upgrade Strategy for SHC

[tgarg-splunk]

Implemented the design for the SHC upgrade strategy for the Splunk Operator Level II support. Specifically:

• Added isSearchHeadReadyforUpgrade to check if MC is ready before a version upgrade 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