Split out fluent builder generators and clean up (#3585)

It looks like I will need to render specialized fluent builders for
waiters. This PR is some prerequisite work to split fluent builder
generation out of the fluent client generator into its own thing. This
makes it possible to refactor the fluent builder generator to be more
configurable in a follow up PR so that it can generate waiter fluent
builders as well.

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Co-authored-by: Zelda Hessler <zhessler@amazon.com>