[native_assets_cli] Non-predefined targetOS

[dcharkes]

We should support an open ended OS-list.

• Flutter tools wants to move away from having a predefined list of OSes.
• There are other embedders (non-Dart-standalone, non-Flutter) that run on other OSes than the 5 predefined OSes.

Currently the JSON format uses a string for target OS, so this is extensible.

However, the Dart API currently has only the 5 predefined OSes available. We should make this extensible in the API.

This is related to working with open ended asset types, or open ended JSON format for config/output in general:

• [native_assets_cli][native_assets_builder] CLI input and output composability #853

Secondly, helper packages should also be extensible. For example a package that has C source code and uses native_toolchain_c should be able to somehow build for these other OSes. There is probably multiple options for this:

1. Make PRs for such packages to support your own OS.
2. Ensure packages have some kind of callbacks/pluggability to add support for other OSes. (But how would that work? Dependency injection?)

Thanks for the input @johnmccutchan!