Add oauth2 support for scalar

[DenuxPlays]

Is your feature request related to a problem? Please describe.

oAuth2 works great with Swagger but in scalar I have to manually define client id and client secret every time.

Describe the solution you'd like

The same configuration parameters as swagger:

    oauth:
      client-id: financrr
      client-secret: 6K0CtYFXexS0MsuHNLYruJk8G4MsP5oZ
      use-pkce-with-authorization-code-grant: true

Describe alternatives you've considered

Just pasting id and secret in every time.

Additional context

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[hanspagel]

Related issue here: #3087

Relevant upstream PR: https://github.com/scalar/scalar/pull/7145