Allow to override the local authentication mock

**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)](https://learn.microsoft.com/en-us/azure/active-directory-b2c/best-practices#operations)

Start at the `swa start` entry point and trace how the local authentication mock is selected and configured. Define the command-line argument or configuration setting needed to use an existing Azure AD B2C tenant, then verify that local development uses that tenant instead of the default mock.

Written by the indexing model from the issue text.