remove Get<T> extension method to make tests more clear

[shiftkey]

Fixes #1062

As part of #1062 I found that an extension methods for Get<T>(uri) is interfering with some of our unit tests - as it's an extension method contributors can't verify it was invoked correctly, and when that doesn't work blindly add the second parameter to get the test to pass.

This is done in a lot of tests, and it's a trivial extension, so why not just make it a feature of the interface?

I'm happy to leave the other methods in ApiExtensions as-is, they're not used as broadly.

cc @haacked for feels and any potential quirks about switching from extension method to interface method.

[haacked]

The only downside is that if you implement the interface, you only need to implement the one Get method and you know the extension method is implemented correctly with regard to that one method. Now, implementers have to implement both and implement them correctly.

Having said that, this really does clean up the tests and API and reduces pain and it's a really really simple method so I'm 👍

[shiftkey]

@haacked yeah, agreed on the need to implement this explicitly. If we had multiple implementations I'd probably explore something else...