Code-generate Acceptors for waiters

[jbelkins]

Every Smithy waiter will include a list of Acceptors, which define the conditions that cause a change in waiter state.

Code-generate a Swift expression creating the Acceptors that are defined in a waiter, and make them available as a static variable that contains an array of Acceptors.

Requirements:

• Code generate all possible forms of Acceptor, including JMESPath expressions
• Create a static variable containing an array of Acceptors for each defined waiter
• Include unit tests ensuring that code is generated correctly for various Smithy definitions

When complete, this work should merge to a Waiters project branch while awaiting the completion of the rest of the Waiters project.