[Security Solution][Entity Analytics][Threat Hunting]: Add entity threat hunting feature flag (elastic#241918)

## Summary

- add experimental feature flag for entity threat hunting experience
- default the new flag to disabled until rollout planning

### Checklist

Check the PR satisfies following conditions. 

Reviewers should verify this PR satisfies this list as well.

- [ ] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)
- [ ] Review the [backport
guidelines](https://docs.google.com/document/d/1VyN5k91e5OVumlc0Gb9RPa3h1ewuPE705nRtioPiTvY/edit?usp=sharing)
and apply applicable `backport:*` labels.

Author: abhishekbhatia1710

x-pack/solutions/security/plugins/security_solution/common/experimental_features.ts

@@ -76,6 +76,11 @@ export const allowedExperimentalValues = Object.freeze({
    */
   entityDetailsHighlightsEnabled: false,
 
+  /**
+   * Enables the experimental Threat Hunting home experience.
+   */
+  entityThreatHuntingEnabled: false,
+
   /**
    * disables ES|QL rules
    */