Use AzureCLI / Browser credentials in `darc` when talking to BAR

[premun]

• Users can now omit the -p argument (bar password/token) and credentials will kick in.
• Providing the -p will override this and the token provided will be used - this makes it a non-breaking change and will also allow users to get the Entra token on their own time.
• darc authenticate no longer stores the BAR token.

https://github.com/dotnet/dnceng/issues/2694

Release Note Category

• [x] Feature changes/additions
• [ ] Bug fixes
• [ ] Internal Infrastructure Improvements

Release Note Description

darc does not require BAR token anymore

[premun]

I think this should not get merged yet because we need to make sure this preserves the BAR token in darc authenticate so that people can keep using darc before they are assigned on the Entra app (which doesn't exist for production)

[premun]

This needs more work still