Namespace ZSS storage keys per authenticated user

[taban03]

Proposed changes

dataService->localStorage/remoteStorage are shared by every caller of
this plugin. serveStorage bound get/set/delete directly to the raw
URL key with no ownership check, so any authenticated user could
read, overwrite, or remove another user's entry via
GET/POST/DELETE /zssStorage/.

Keys are now namespaced as ${request->username}:${key} before
reaching storageGetString/storageSetString/storageRemove.
request->username is set by the http server itself after successful
auth (session token, JWT, or client cert), never client-controlled.
Requests with no authenticated username are rejected instead of
falling back to an un-namespaced key.

This PR addresses Issue: [Link to Github issue within https://github.com/zowe/zlux/issues if any]

This PR depends upon the following PRs:

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Change in a documentation
• Refactor the code
• Chore, repository cleanup, updates the dependencies.
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

PR Checklist

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• If the changes in this PR are meant for the next release / mainline, this PR targets the "staging" branch.
• My code follows the style guidelines of this project (see: Contributing guideline)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes
• video or image is included if visual changes are made
• Relevant update to CHANGELOG.md
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works, or describe a test method below

Testing

Further comments

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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