Add basic managed logging endpoint support to go-fastly

[jaredmorrow]

Support enabling and disabling managed logging for a service.

[Integralist]

@jaredmorrow I'm guessing this error is because you need to have that feature enabled on your account for the VCR fixture to work? I stand corrected, I just reviewed the changes and looks like the fixtures were generated correctly. 🤔

--- FAIL: TestClient_ManagedLogging (0.21s)
    managed_logging_test.go:27: 401 - Unauthorized:
        
            Title:  Provided credentials are missing or invalid

[jaredmorrow]

@Integralist That was my initial concern / thought as well. I didn't think it was needed, but I still added the test service 7i6HN3TK9wS159v2gPAZ8A to the feature in question in Fastly App -> Admin.

[jaredmorrow]

@Integralist it was a case issue. I switched to underscore to match the rest of the fixtures. Now it works.

[Integralist]

Thanks @jaredmorrow for opening this PR (and thank you for your patience waiting for a review!). I've a couple of comments/suggested tweaks.