OAuth2 Authorization Code Grant with PKCE Flow auth.py

[dean-taylor]

Is your feature request related to a problem? Please describe.
Supporting deployment of MONAI Label at scale, providing efficient resource utilisation and implementing controls around resource and data access. Common industry practice for API driven workloads is to utilise OAuth2 and with the upcoming OAuth 2.1 specification it would require the "OAuth2 Authorization Code Grant with PKCE".

Describe the solution you'd like
auth.py already includes OAuth2 basic authentication. I would like this to extend to OAuth2 Authorization Code Grant with PKCE.

Describe alternatives you've considered
I am not aware of alternative solutions to provide authz for API driven workloads that integrate at a level of OAuth at this time.

Additional context