Refactor synthetic traits to use builtin input and output traits

[aajtodd]

Next version of smithy (1.15+) will have support for builtin input and output traits. We currently generate these as synthetic traits and should make use of the new functionality to be forwards compatible with Smithy 2.0 (rules around input and output structures will be different than other structures in 2.0 regarding @required or default value)

This also introduced a new Unit type which signifies an operation really truly doesn't have an input or output. It will be considered a breaking change to go from Unit to having an input or output which will allow us to generate operations targeting Unit without a synthetic input or output type.

[ianbotsf]

We already have a customization in place to deal with synthetic shape generation.