Adds waiter support to smithy-go

[skotambkar]

While reviewing, please also review the generated waiters for weather service: https://gist.github.com/skotambkar/73f4f0825f9960fa29834753f1f59c30. (updated)

Description of changes:

• Adds support for waitable trait.

TODO:
[x] move waiter middleware to be waiter-specific and code generated. This is required to remove the use of empty interfaces on waiter state mutator options.
[x] Test waiter generation with weather service.

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[jasdel]

Probably https://gist.github.com/skotambkar/e4d4ff1404df8f78c598a9471f9212a4#file-listcitieswaiter-go-L7-L32 needs a APIOptions member to provide additional api options like stack mutators.