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

• https://github.com/wso2-extensions/identity-governance/pull/870
• https://github.com/wso2/carbon-identity-framework/pull/6046

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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/wso2/identity-apps/pull/7010/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2) | Coverage Δ | | |---|---|---| | [@wso2is/core](https://app.codecov.io/gh/wso2/identity-apps/pull/7010/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2) | `31.79% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2#carryforward-flags-in-the-pull-request-comment) to find out more.

[Yoshani]

UI:

[wso2-jenkins-bot]

🦋 Changeset detected

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

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