WPS: added key byte cache to the auth policy

[KrzysztofCwalina]

cc: @jkotas

At some point we might want to cache the whole token, but that is a bit more tricky as the JWT audience changes from call to call (it's the absolute URI).

[jsquire]

Should we consider adding a test or two for validation of the behavior?

[jsquire]

Suggested change

	KeyBytes = Encoding.UTF8.GetBytes(key);
	KeyBytes = string.IsNullOrEmpty(key)
	? Array.Empty<byte>()
	: Encoding.UTF8.GetBytes(key);

Probably a tiny efficiency, but should we consider avoiding GetBytes when the string is empty? It looks like there's no special case for it and it'll do some stack allocations that we can avoid. (ref) Maybe a factory method for an empty instance, instead?

[KrzysztofCwalina]

Should we consider adding a test or two for validation of the behavior?

We will be adding test before GA but it's a non trivial workitem. The problem is that this component is inherently difficult to test. There are not APIs to observe changes to the service.

I was also thinking about unit testing this policy with fake messages. I need to talk to Pavel about adding fake Transport/Request/Response for that. He wanted to do something like that in Azure.Core, i.e. add test only Request/Response.