[pgAdmin4][Patch]- Feature #7012 - disable master password requirement when using alternative auth source

Hi,

Please find the attached patch to implement the feature #7012 - Disable
master password requirement when using alternative auth source

When pgAdmin stores a connection password, it encrypts it using a key that
is formed either from the master password, or from the pgAdmin login
password for the user. In the case of auth methods such as OAuth, Kerberos
or Webserver, pgAdmin doesn't have access to anything long-lived to form
the encryption key from, hence it uses the master password. And if the
master is disabled, there is no way to store the connection password.

To resolve this, we have added an option to config.py (which defaults to
None) for an alternate encryption key. pgAdmin would use this if a) the
master password is disabled AND b) there is no suitable key/password
available from the auth module for the user. If the option is set to None,
pgAdmin works as it does now.

Thanks,
Khushboo