Add section on working with the user's `ClaimsPrincipal` in code

[guardrex]

Description

Noticed from looking at dotnet/aspnetcore#55327 that we could have a short section and example. Beyond just covering it explicitly, it's also used in examples here and elsewhere.

Page URL

https://learn.microsoft.com/en-us/aspnet/core/blazor/security/authentication-state?view=aspnetcore-7.0

Content source URL

https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/blazor/security/authentication-state.md

Document ID

bf7b7fe3-ad57-5ee7-04e3-0d73f288b005

Platform Id

8b31ddf9-54d6-9d42-5052-105cc49413af

Article author

@guardrex

Metadata

• ID: bf7b7fe3-ad57-5ee7-04e3-0d73f288b005
• PlatformId: 8b31ddf9-54d6-9d42-5052-105cc49413af
• Service: aspnet-core
• Sub-service: blazor

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