Authentication follow-ups

[captainsafia]

• Figure out service registration behavior when multiple schemes are registered via builder.Authentication (Add dotnet user-jwts tool and runtime support #41520 (comment))
• Support reading default scheme from config
• Provide AddConfiguration method on AuthenticationBuilder
  - [ ] Investigate moving IsAuthenticationConfigured state to AuthenticationBuilder API (Add dotnet user-jwts tool and runtime support #41520 (comment))
  - [ ] Centralize schema keys used in CLI tool and runtime
• Make secrets retrieval more robust (Add dotnet user-jwts tool and runtime support #41520 (comment))
• Add unit tests for configure options
• Support IOptionsMonitor for ConfiureAuthOptions

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We're moving this issue to the .NET 7 Planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s).
If we later determine, that the issue has no community involvement, or it's very rare and low-impact issue, we will close it - so that the team can focus on more important and high impact issues.
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