add 'VERIFY_AND_CHANGE_EMAIL' linkType for generateEmailActionLink

[utamori]

Summary

• Add support for VERIFY_AND_CHANGE_EMAIL linkType in Firebase Auth email action links
• Implement VerifyAndChangeEmailLink and VerifyAndChangeEmailLinkWithSettings methods
• Add comprehensive tests for the new functionality including tenant integration tests

Changes

• Added verifyAndChangeEmail linkType constant
• Updated generateEmailActionLink to accept optional newEmail parameter
• Added new public methods for verify and change email functionality
• Updated existing methods to use the new signature
• Added unit tests and integration tests

Test plan

• [x] Unit tests for new functionality
• [x] Integration tests for tenant-aware scenarios
• [x] Existing tests continue to pass

This addresses the feature request in #470 by providing the ability to generate email action links for email verification and change flows.