`AuthContext.IsPeerAuthenticated` should return true if `User.Identity.IsAuthenticated` does

### Is your feature request related to a problem? Please describe.
After _loads_ of trial & error (no offense, but some docs are confusing for beginners), it seems my Blazor WASM standalone app authenticates Azure B2C users via MSAL correctly - except for 1 thing:
`context.AuthContext.IsPeerAuthenticated` always returns `false`, even if `context.GetHttpContext().User.Identity.IsAuthenticated` returns `true`.

Since the docs for `AuthContext.PeerIdentityPropertyName` state:
> Gets the name of the property that indicates the peer identity

… I looked at the code which clarified that `IsPeerAuthenticated` returns `true` as soon as the former just isn't `null` - but that doesn't always seem to get set properly.
From what I could find (mainly 2 unit tests in this repo), the C# implementation solely focuses on authentication via certificates, because if the underlying `HttpContext` contains a `ClaimsPrincipal` with an `IIdentity` whose `IsAuthenticated` is true, gRPC seems to "simply not care".

### Describe the solution you'd like
Please, for Padawan-like developers like me, i.e., those struggling with authentication & authorization, enable that authenticated users (= those with an `HttpContext` containing a `ClaimsPrincipal` whose `IIdentity.IsAuthenticated` evaluates to `true`), can be easily checked/identified via `context.AuthContext.IsPeerAuthenticated`, too.
Because otherwise `context.GetHttpContext()` needs to be called on each & every method call.

### Describe alternatives you've considered
Continuing to use either workaround:
- Calling `context.GetHttpContext().User.Identity.IsAuthenticated` inside each overwritten method.
- Using a custom `AuthenticationInterceptor` which overrides 9 (!) methods to evaluate the `IIdentity` property & in the event of it being `true` sort of "modifies" the existing `ServerCallContext` by cloning everything from it, except for the `AuthContext` which is replaced with one that has `IsPeerAuthenticated` return `true`.

### Additional context
The docs regarding `AuthContext` should be improved to make its content as well as usage clearer - maybe even by adding a usage example for the scenario I just described.

Start by reading AuthContext.IsPeerAuthenticated, AuthContext.PeerIdentityPropertyName, and the two existing unit tests mentioned in the issue. Check how HttpContext.User.Identity.IsAuthenticated relates to the current certificate-focused behavior. Done means authenticated ClaimsPrincipal identities are recognized and the AuthContext documentation explains the behavior with a usage example.

Written by the indexing model from the issue text.