Regenerated Clients with Paginator Support

[skmcgrail]

Adds support for paginators.

Related #820
Related aws/aws-sdk-go#3513
Closes #439

Depends on:
aws/smithy-go#234
smithy-lang/smithy#628

[jasdel]

What do you think about the following as a way we can simplify some of the logic around that Limit option value.

The Paginator should auto fill Limit option based on the the value in the Input struct. So if a user does use the func option for setting the Limit option it would start off in what ever value was in the Input parameter if there was one.

This strategy highlights a question: What if user wants to clear the page size limit via the Limit option? How would this be done? Would Limit option value of 0 be translated to be the zero value of a boxed limit input member?

// For boxed PageSize
var options PaginatorOptions
options = ptr.ToInt32(input.PageSize)


// For unboxed PageSize
var options PaginatorOptions
options.Limit = input.PageSize

// if PageSize is boxed
if p.options.Limit == 0 {
     input.PageSize = nil
} else {
    input.PageSize = ptr.Int32(p.options.Limit)
}

// if PageSize is unboxed
input.PageSize = p.options.Limit

The above has one notable downside it is impossible to send a page size of 0 with this strategy. Arguably that probably is OK, because the behavior isn't really well defined if 0 were to be sent, Could be failure be service, could be infinite loop, service could ignore page size and use a default.