all: find a better way to set a custom HTTP client

[rakyll]

Our current method of requiring users to pass an HTTP client in the current context is an abuse of context. We should find a better way set a custom HTTP client and deprecate the current method.

We can add a HTTPClient field to the Config types, but there are cases like AppEngineTokenSource where there is not a Config type and we have to break the current APIs to improve the situation.