Adding new helper functions to internal.HTTPClient API

[hiranya911]

The pattern of making a JSON HTTP request, parsing the response and handling errors is very common in the SDK. This PR introduces a new DoAndUnmarshal() helper function to internal.HTTPClient to implement this pattern. Also includes:

1. Support for centralized error handling via CreateErrFn and SuccessFn options.
2. Deprecated the existing ErrParser option, and the CheckStatus() and Unmarshal() functions on the Response.

[bklimt]

It's kinda weird for both HTTPClient and Request to have so many fields in common. Would it make sense to package them into a separate struct that they each get a pointer to?

[hiranya911]

The problem with that is, callers have to use another level of nesting to specify common properties:

req := &internal.Request{
  CommonSettings: &internal.CommonSettings{
    SuccessFn: mySuccessFn,
  },
}

I'd prefer following over that:

req := &internal.Request{
  SuccessFn: mySuccessFn,
}

[bklimt]

That's fair.

I do feel like these http methods have probably gotten more complicated than they need to be, and maybe we got some abstraction wrong that's making thing needlessly difficult. But I don't have any specific objections to this change. Certainly nothing worth holding this PR up any longer.

But I think we should brainstorm how to clean this up at some point.

[bklimt]

LGTM