added: command plugin to download model files.

[Arpit160399]

This pull request addresses issue #62.

Changes Made:

• Introduced a version-specific Swift package file to facilitate plugin capability for Swift toolchain 5.9 and above.
• Added a new command plugin within the service to enhance functionality.

Additional Notes:

I attempted to implement gatekeeping using availability, but network permissions need to be set at the package level, which is only available from Swift toolchain 5.9 onward.

[adam-fowler]

I've added some comments

[adam-fowler]

In Soto (as with SwiftNIO) we look to support the last three versions of Swift. So for the moment that'd be 5.8, 5.9 and 5.10. Currently with your setup the download plugin is only available for 5.9, not 5.10. I would instead create a Package.@swift-5.8.swift which is a copy of the current Package.swift and make this version the default Package.swift

[Arpit160399]

So, with the current package setup, if the current Swift version doesn't match any version-specific manifest, the package manager will pick the manifest with the most compatible tools version. This means the package manager will pick Package.swift for Swift 5.8 and Package@swift-5.9.swift for Swift 5.9 and above, as its tools version will be most compatible with future versions of the package manager. I read this in the documentation here.

In terms of readability and maintainability, I agree with your approach of keeping the 5.8 version tag instead of 5.9.