[Enhancement proposal]: [WIP] Add Swift Package Manager support

[engels-all-win-software]

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Proposal

This issue aims to track the progress of implementing SPM support within permission handler

This SPM implementation is still in development due to differences between CocoaPods and SPM. Preprocessor macros in Swift Package Manager require additional work to streamline the process of defining permissions, similar to how we did it in the Podfile.

This implementation maintains CocoaPods alongside SPM, and the example app uses CocoaPods. To use the SPM integration, simply remove the Podfile and execute pod deintegrate. Then, enable SPM support and run the app again.

I would like to coordinate with the team and ask if there's an easy or ideal way to implement the preprocessor macros so these changes won't negatively affect the developer experience. Since Swift Package Traits are still a work in progress and it's uncertain whether the feature will be implemented in Swift, we need to explore alternative solutions in the meantime.

I've taken the approach of activating permission modules through environment variables, similar to what's mentioned here: Conditionally compile code in a Flutter plugin using Swift Package Manager. This simplifies the process. One useful option is creating an xcconfig file that can be imported into the Release or Debug xcconfig files. This file would contain the desired permissions declared as variables. This would be a one-time setup and wouldn't require exporting those variables each time the app is run.

Here are some useful links with additional information:

• [SwiftPM] Support conditional compilation in plugins
• Swift Package Traits Feature Request
• Conditionally compile code in a Flutter plugin using Swift Package Manager

Pitch

Implementing Swift Package Manager support will improve the developer experience and keep us in sync with the latest Flutter upstream changes.