[FR] Support VERIFY_AND_CHANGE_EMAIL in generate_email_action_link (parity with firebase-admin-node)

Is your feature request related to a problem? Please describe.

The Identity Toolkit accounts:sendOobCode endpoint supports a VERIFY_AND_CHANGE_EMAIL request type that generates an OOB link which, when followed, both verifies the new address and updates the user's email. This is the recommended way to implement a secure "change email" flow — the verification link is sent to the current address so the flow isn't vulnerable to a stolen session setting a new email without the original owner's knowledge.

The Python Admin SDK cannot generate these links today. firebase_admin/_auth_utils.py declares:

VALID_EMAIL_ACTION_TYPES = set(['VERIFY_EMAIL', 'EMAIL_SIGNIN', 'PASSWORD_RESET'])

validate_action_type() rejects 'VERIFY_AND_CHANGE_EMAIL' before the request is made, and there is no new_email parameter on generate_email_action_link to carry the target address.

Describe the solution you'd like

Mirror the API added to firebase-admin-node in firebase/firebase-admin-node#1633 (merged April 2022):

1. Add 'VERIFY_AND_CHANGE_EMAIL' to VALID_EMAIL_ACTION_TYPES.
2. Add an optional new_email: str | None = None parameter to generate_email_action_link (and the corresponding Client / auth module methods), required when action_type == 'VERIFY_AND_CHANGE_EMAIL'.
3. Include newEmail in the accounts:sendOobCode payload when set.

For reference, the Node signature is:

generateEmailActionLink(requestType, email, actionCodeSettings?, newEmail?)

Describe alternatives you've considered

Calling accounts:sendOobCode directly through Client._user_manager._make_request(...) with {"requestType": "VERIFY_AND_CHANGE_EMAIL", "email": ..., "newEmail": ..., "returnOobLink": True, ...}, reusing firebase_admin._user_mgt.encode_action_code_settings for ActionCodeSettings serialization. This works but depends on SDK internals (private _user_manager, private _make_request, private encode_action_code_settings) and bypasses the SDK's validation and error mapping.

Additional context

• Node SDK PR adding the same feature: firebase/firebase-admin-node#1633
• The Identity Toolkit requestType enum already includes VERIFY_AND_CHANGE_EMAIL; the REST surface is unchanged.
• Verified against firebase-admin 7.4.0.