Allow providing a custom HttpMessageHandler

[czietsman]

I would like to be able specify a custom HTTP message handler so that I can capture the 'intuit_tid' header and add it to our logs.

I've added an optional HttpMessageHandler innerHandler = null where it made sense and passed it through.

The following decisions were made:

• custom handler was not added to DiscoveryClient(string discoveryUrl), since there already exists another constructor DiscoveryClient(string authority = OidcConstants.Discovery.IssuerUrl, HttpMessageHandler innerHandler = null) which would conflict
• custom handler was not added to TokenClient in the constructor for RefreshTokenHandler, since there exists an overload where a TokenClient can be passed to the RefreshTokenHandler

[nimisha84]

Thanks for the PR. I had moved to a different team and SDK ownership has been transferred. CLosing off the pending PRs. Looks like with this change, if existing devs directly call the different oauth lib clients, they can break. I would suggest creating another constructor instead of updating the existing one for your use case. Please update us when you are ready with the PR.