Add support for custom authentication handlers

Today if you try to overwrite a service that we've registered, it can cause problems. We have a fairly vague explanation of this here: https://docs.microsoft.com/en-us/azure/azure-functions/functions-dotnet-dependency-injection#overriding-host-services.

Customers that try to add their own authentication in a FunctionsStartup can run into odd errors later like

```
System.InvalidOperationException : No authentication handler is registered for the scheme 'ArmToken'. The registered schemes are: WebJobsAuthLevelIdentityServerAuthenticationJwt, WebJobsAuthLevelIdentityServerAuthenticationIntrospection, WebJobsAuthLevel, BearerIdentityServerAuthenticationJwt, BearerIdentityServerAuthenticationIntrospection, Bearer. Did you forget to call AddAuthentication().Add[SomeAuthHandler]("ArmToken",...)?
```

One example would be doing this in your startup:

```
builder.Services.AddAuthentication()
.AddCookie();
```

We should add a way to allow auth handlers from Startup compose with the built-in functions handlers. Or another alternative may be to prevent custom handlers from impacting `/admin` routes.

Start by reviewing the FunctionsStartup example using AddAuthentication().AddCookie() and the linked dependency-injection documentation. Compare the built-in handlers and the /admin routes described in the issue, then define which composition behavior should be supported and how completion would be verified.

Written by the indexing model from the issue text.