Always run MsalSilentBearerTokenProvider to provide cached and refreshed tokens

[JohnSchmeichel]

Previously in ADAL AdalCacheBearerTokenProvider would be skipped if IsRetry was true, but rely on WindowsIntegratedAuthBearerTokenProvider to provide a cached token. In the MSAL variants, MsalSilentBearerTokenProvider would also be skipped if IsRetry was true, but in this case MsalWindowsIntegratedAuthBearerTokenProvider does not provide cached tokens, leading to a prompt.

In MSAL the AcquireTokenSilent calls obtain cached or refreshed tokens for both brokered and non-broker cases, and we should rely on the fact that this will always return a valid token. So MsalSilentBearerTokenProvider should always be run to provide cached tokens.

The context here is that we've seen some bad cases where NuGet clients are dropping or invalidating access tokens, getting Unauthorized (401) responses from external endpoints, then calling back into the credential provider for new tokens. These cases require both the token cache and/or silent authentication to be enabled otherwise users get constant prompts during package restore.