added account linking/email existence check function

[home0712]

Tasks Completed:

• added a function to link a google account to an existing email/pwd account
• added a function to check if the entered email already exists in the database (use for signing up process)

p.s.
I guess I had a misunderstanding regarding account linking and authentication. I am sorry for any confusion caused.
Based on the potential scenarios, I have implemented the code as follows:

1. A user with an existing email/pwd account ([email protected]) on the web tries to sign in (or sign up) with a google account using the same email address ([email protected]).
   - the accounts are automatically linked according to the firebase auth setting.
2. A user with an existing google account ([email protected]) on the web tries to sign up using an email/pwd account method with the same address ([email protected]).
   • this may be prevented if the entered email is checked during the sign-up process.
3. A user with an existing email/pwd account ([email protected]) on the web wants to link a google account ([email protected]).
   - account linking is only applicable in this case
   - it is not allowed to link an existing Google account to a new Google account.