Fixes issue using AAD credentials with EventHubs.

[mitchdenny]

I've discovered what I think is an error in the scope value that the EventHubs client is passing to token providers when it requests a token. This incorrect value affects AAD-based credential types such as ClientSecretCredential and ManagedIdentityCredential. I didn't test the later but I'm pretty confident it'll stop any AAD based credential from working with EventHubs - so I think we probably need to ship an update to the EventHubs library.

I verified that the capitalization of Azure was the issue in the scope value by creating a man-in-the-middle token provider which substituted the scopes when fetching a token to compensate for the error. If you want a repro here you go:

public class MimCredential : ClientSecretCredential
    {
        public MimCredential(string tenantId, string clientId, string clientSecret) : base(tenantId, clientId, clientSecret)
        {
        }

        public override Task<AccessToken> GetTokenAsync(string[] scopes, CancellationToken cancellationToken = default)
        {
            scopes[0] = scopes[0].ToLower(); // HACK!!!!
            return base.GetTokenAsync(scopes, cancellationToken);
        }
    }

I also removed the (what I think) is a redundant backslash).

[jsquire]

Thanks!