[pgAdmin][Patch] - RM #5940 - Add support for Oauth 2 authentication

Please find the attached patch for the RM #5940 - Add support for Oauth 2 authentication. 

The patch includes:

- Pluggable Oauth 2 authentication support with the multiple providers

- Configurable support for Master Key in Oauth2 and Kerberos authentication, so user can save the database server password

- Introduced the separate module/blueprint for kerberos and Oauth2 under authentication module for the readability and maintainability

Note: The initial patch for Oauth2 authentication was sent by Florian Sabonchi, I have worked on top of it.

Thanks,

Khushboo