Add an option to AuthorizationMessageHandler to not throw if unable to get token

## Context
When using `HttpClient` to make API calls from Blazor WebAssembly application, users have to add [AuthorizationMessageHandler](https://github.com/dotnet/aspnetcore/blob/main/src/Components/WebAssembly/WebAssembly.Authentication/src/Services/AuthorizationMessageHandler.cs) to the configured HttpClient instance. That's required specifically for calling APIs which require authorization.
What the AuthorizationMessageHandler does, it will try to get an access token through the configured token provider and send it with the outgoing HTTP Request headers. And if the token provider can't retrieve a token (let's say for a non-authenticated user), the corresponding http client request will fail with `AccessTokenNotAvailableException`.

## Problem
Users who intend to call an unprotected API (one which doesn't require authorization) don't need to use the AuthorizaitonMessageHandler. Hence, in Blazor WebAssembly applications, which have both protected and unprotected APIs to communicate with, users currently have to have two separate HttpClient instances to work with: one configured with AuthorizaitonMessageHandler, and another which does not.

The problem, however, arises, when the user wants to interact with an API, which doesn't require the callers to be authorized, but behaves differently if they are. For this scenario, the current solution doesn't work, and users will have to avoid using AuthorizaitonMessageHandler and inject tokens directly into their outing http requests manually, which defeats the purpose of AuthorizaitonMessageHandler.

## Proposal
Expose a mechanism to configure the behavior of the AuthorizationMessageHandler, so that it doesn't throw `AccessTokenNotAvailableException` if token can't be aquired. Isntead, it lets the call throuigh without attaching an access token to that request.

The current behavior should remain as the default, and customers who would like to use the above proposed behavior will need to explicitly opt-into it by setting some boolean parameter.