Add OOB flow for VERIFY_AND_CHANGE_EMAIL

[naoina]

Description

Implement VERIFY_AND_CHANGE_EMAIL OOB flow to accounts:sendOobCode API in Firebase Emulator. This changes allows you to use verifyBeforeUpdateEmail in Firebase Javascript SDK, generateVerifyAndChangeEmailLink in Firebase Admin SDK and accounts:sendOobCode API with requestType VERIFY_AND_CHANGE_EMAIL as well.

Refs #3424

Scenarios Tested

• Tested the default handler for verifyAndChangeEmail mode by accessing the URL returned from generateVerifyAndChangeEmailLink
• Tested the custom email handler using checkActionCode and applyActionCode of firebase/auth

[joehan]

Thanks for the contribution! LGTM, but I'd like @aalej to review as well, since he's done some work in this area of the codebase recently.

[aalej]

This LGTM! Thanks for submitting a PR! Tested using https://github.com/aalej/pull-7618/tree/main and it looks like the behavior in the emulator matches the one in prod. @joehan, this looks like a new feature, we might need to find someone from Auth team to review this as well.