Drop gyb in favor of swift macros

[Amzd]

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We can use swift macros to generate the code that's currently generated by gyb.

Pros

• Code completion and type checking on types that will get generated (rn they are gyb files which get nothing)
• No extra step to run when editing types that have generation step, just swift build.
• No dependency on gyb

Cons

• Requires Swift 5.9 so we will have to up the requirement from currently 5.5

Should be fun project for anyone with decent knowledge of Swift who wants to learn about macros.