Add support for setting swift tools version

[ttypic]

It will be nice to extract swift tools version to swiftklib extension config.

For now it always hardcoded to 5.5

[markst]

Assuming reason would be this opens up features enabled as part of newer swift versions?

I wonder if my comments here are relevant:
#10 (comment)

As if the requirement for using the plugin depended on defining the Package.swift it would mean the owner defines the swift tools version ?

[markst]

Not sure how relevant but it is also possible to compile Swift packages to Android:

.target(name: "Android", condition: .when(platforms: [.android]))

Hope you don't mind me posting this here too:
https://github.com/scade-platform/swift-android-examples