fix issues with Get/SetAttributes Go code generation

[jaypipes]

This patch adds to the code generator for the SDK linkage for APIs that
use the SetAttributes-flavor of updating a resource. Previously, the
sdk.go files for these APIs had empty sdkUpdate operations with a
big TODO that ended up breaking the service controller (quite
predictably).

While implementing this patch, I stumbled upon yet another unexplainable
inconsistency between two SetAttributes operations within the SNS
service API. The SNS SetPlatformApplicationAttributes API call accepts a
field in its Input shape called Attributes that is a map of key/value
pairs for attributes to set on the platform application. Unfortunately,
the similarly-named SNS SetTopicAttributes API call apparently does
NOT work the same way. Instead, there is a single AttributeName and
AttributeValue field in the Input shape and you need to call the
SetTopicAttributes API call once for each modified attribute. :(

In fact, the [official documentation][0] for the SNS SetTopicAttributes
API call says that the AttributeName field in the Input shape is
actually a "map of attributes with their corresponding values". But
that isn't actually the case...

So, the code in this patch for now just contains a TODO in the
CRD.GoCodeSetAttributesSetInput() method for SetAttributes APIs that
can only operate on a single attribute at a time. We will use the
CustomOperation functionality as a temporary workaround for these APIs
while we come up with a more permanent code-generated solution.

There were a series of problems that I uncovered when investigating the
cause of Issue #296:

• The code returned from GoCodeGetAttributesForOutput was failing to set
  Status.ACKResourceMetadata.ARN and
  Status.ACKResourceMetadata.OwnerAccountID when the attribute fields
  corresponded to the primary resource ARN or the owner ID. This was the
  direct cause of Issue sns: GENERATION FAILURE! there's a required field PlatformApplicationArn in Shape GetPlatformApplicationAttributesInput that isn't in either the CR's Spec or Status structs #296
• The code in templates/pkg/crd_sdk.go.tpl that ran at the end of the
  sdkCreate() call was inadvertently overwriting any setters of
  Status.ACKResourceMetadata that had previously executed
• The code returned from APIs with GetAttributes operations was always
  returning a nil-guard and constructor for the ACKResourceMetadata
  struct, even when no attribute fields actually set either ARN or
  OwnerAccountID. When I removed the check in the template for Status
  fields being required for the resp variable to be defined, this was
  causing "resp" variable unused compilation failures

Issue #296

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[a-hilaly]

/lgtm 👍
Was fun reading the comments :)