spike: investigate ways to support packages which hook into the codegen step

[felangel]

As a developer, I want to be able to write a Dart Frog package which is able to integrate into the Dart Frog build step and affect the generated output so that I can provide valuable functionality without having to contribute to the core Dart Frog package directly.

Examples:

Client Generation: it would be nice to be able to create a separate, standalone package to handle client generation in Dart Frog without having to bloat the core package

Open API Generation: it would be nice to be able to create a separate, standalone package to handler open api documentation generation in Dart Frog without having to bloat the core package

WebSockets: if this capability were available, it would allow for improvements to the Dart Frog WebSocket API like:

FutureOr<void> onSocketConnection(channel, protocol) async {...}

[renancaraujo]

Hello, after some research I created this design doc with some gameplan for to happen.

In summary, it is possible to create a plugin system for Dart Frog.
Dart Frog Cli would then be responsible for registering and running plugin code on build and dev.

[tomarra]

We have the documentation written up for this so closing this as "Complete". Need to figure out prioritization next.