[Feature]: Use Swift Package Plugins for codegen (server driven workflows)

[Tyler-Keith-Thompson]

What prompted this feature request?

Our current approach to create an aggregator for WorkflowDecodable is a little cumbersome. It involves running a standalone CLI utility.

Describe the solution you'd like

If we used swift package plugins we could integrate into the build system better, meaning people can set it and forget it.

Describe alternatives you've considered

Keep doing it the way we are?

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