Allow to override the local authentication mock

[noontz]

Is your feature request related to a problem? Please describe.
In local development I would like to test against an existing Azure AD B2C tenant created specifically for this purpose

Describe the solution you'd like
I would like a simple swa start --argument / configuration setting to override the default local authentication mock

Describe alternatives you've considered
I am not aware of any alternatives, but this might be possible already

Additional context
Azure AD B2C operations best practice (first row)