FR: add 'VERIFY_AND_CHANGE_EMAIL' linkType for generateEmailActionLink

[tyahha]

environment

• Operating System version: Windows 10
• Firebase SDK version: 4.6
• Library version: 4.6
• Firebase Product: auth

Describe the problem

I want to send custom email when your change email.
And I want to verify email before change email.
Firebase Client SDK have this feature, but Firebase admin sdk have not feature to generate this type email action link.
So, As other email action link types, "EMAIL_SIGNIN", "VERIFY_EMAIL", "PASSWORD_RESET", I want to add feature to generation email action link of "VERIFY_AND_CHANGE_EMAIL" type.

This is an earlier version, but a modified version is available.
tyahha@2d00814

If there are no problems, rebase to the latest version, add the test code, and create a pull request.