Handle username existence and non-existence similarly

[Yoshani]

Purpose

Add option to handle username existence and non-existence in the same way when account verification is enabled. The option Show username unavailability is added.

Related PRs

• Add ShowUsernameUnavailability config to self registration wso2-extensions/identity-governance#870
• Update Preference Retrieval Client to get ShowUsernameUnavailability check value carbon-identity-framework#6046

Related Issues

• Add support to prevent username enumeration in self registration account verification flow product-is#22101

Checklist

• e2e cypress tests locally verified. (for internal contributers)
• Manual test round performed and verified.
• UX/UI review done on the final implementation.
• Documentation provided. (Add links if there are any)
• Relevant backend changes deployed and verified
• Unit tests provided. (Add links if there are any)
• Integration tests provided. (Add links if there are any)

Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines
• Ran FindSecurityBugs plugin and verified report
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 31.79%. Comparing base (90b52b9) to head (4b775a9).
Report is 123 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7010   +/-   ##
=======================================
  Coverage   31.79%   31.79%           
=======================================
  Files          41       41           
  Lines         890      890           
  Branches      204      204           
=======================================
  Hits          283      283           
  Misses        607      607           

Flag	Coverage Δ
@wso2is/core	31.79% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

[Yoshani]

UI:

[wso2-jenkins-bot]

🦋 Changeset detected

The changes in this PR will be included in the next version bump.

Not sure what this means? Click here to learn what changesets are.