add Delta.DifferentExcept() method

[jaypipes]

I need the ability to detect whether there are any differences in a
resource's fields other than a particular one. Basically, in doing the
tagging work for DBInstance, I came across a problem where we were
calling the ModifyDBInstance API call even though there were no changes
to DBInstance Spec fields other than the Tags collection.

So, I need to add the following in an sdk_update_pre_build_request
template for DBInstance:

if delta.DifferentAt("Spec.Tags") {
     if err = rm.SyncTags(ctx, desired, latest); err != nil {
         return nil, err
     }
}
if !delta.DifferentExcept("Spec.Tags") {
     // We don't want to proceed to call the ModifyDBInstance API since
     // no other resource fields have changed.
     return desired, nil
}

This PR adds the delta.DifferentExcept() implementation to enable the
above code.

Issue aws-controllers-k8s/community#1276

Signed-off-by: Jay Pipes jaypipes@gmail.com

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

[RedbackThomson]

Cute little helper method

/lgtm

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[brucegucode]

not too familiar with this code but code logic looks good to me
/lgtm