Swift5/Vapor query parameter coding keys required

Created by: jhheider

The Swift5/Vapor generator doesn't provide a CodingKey for QueryParams, which means they aren't decoded back to their base name. The obvious case is per_page in the spec becoming perPage in the generated code. Without a coding key, it is passed back as perPage in the final query.

(Sorry I didn't bother opening an issue, but this a really small change. Let me know if you want any other formalities followed.)

Technical committee: @jgavris @ehyche @Edubits @jaz-ah @4brunu

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