feat: Ability to specify the authentication type for imap server (#12306)

# Pull Request Template

## Description

This PR adds IMAP authentication mechanism selection to Chatwoot's email
inbox configuration. Users can now choose between 'plain', 'login', and
'cram-md5' authentication methods when configuring IMAP settings,
providing flexibility for different email providers that require
specific authentication types.

https://github.com/chatwoot/chatwoot/issues/8867

The implementation includes:
- Frontend dropdown with numeric keys (1, 2, 3) matching SMTP auth style
- Backend API validation for allowed authentication mechanisms
- Consistent 'cram-md5' format throughout the codebase
- Updated IMAP service to handle different auth types properly

This feature maintains consistency with existing SMTP authentication
options and follows the established UI/UX patterns in the application.

## Type of change

Please delete options that are not relevant.

- [x] New feature (non-breaking change which adds functionality)
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] Breaking change (fix or feature that would cause existing
functionality not to work as expected)
- [ ] This change requires a documentation update

## How Has This Been Tested?

### Manual Testing:
- Tested in Docker environment
- Verified IMAP auth dropdown appears in inbox settings
- Confirmed all three auth mechanisms (plain, login, cram-md5) can be
selected and saved
- Tested API validation by attempting to save invalid auth mechanisms

### Automated Testing:
- Updated existing IMAP service tests to use consistent lowercase values
- Updated API controller tests for authentication parameter handling
- All tests pass locally with the new changes

### Test Configuration:
- Tested with both new and existing inbox configurations

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [x] Any dependent changes have been merged and published in downstream
modules

## Additional Notes

- This feature is backward compatible and doesn't break existing IMAP
configurations
- The 'cram-md5' format is used consistently throughout (UI, API,
storage, services)
- Net::IMAP compatibility is maintained by converting to 'CRAM-MD5'
internally
- Follows the same pattern established by SMTP authentication
configuration

---------

Co-authored-by: João Santos <joao.santos@madigital.eu>
Co-authored-by: Sony Mathew <sony@chatwoot.com>

app/javascript/dashboard/i18n/locale/en/inboxMgmt.json

@@ -1011,7 +1011,8 @@
         "LABEL": "Password",
         "PLACE_HOLDER": "Password"
       },
-      "ENABLE_SSL": "Enable SSL"
+      "ENABLE_SSL": "Enable SSL",
+      "AUTH_MECHANISM": "Authentication"
     },
     "MICROSOFT": {
       "TITLE": "Microsoft",