Proposal: stop having separate FileResourceLoader implementations per platform and rely on FileSystem instead.

[tomsfernandez]

Objective: have a way to unit-test the adapter logic inside the ResourceLoader implementations from a "shared" scala test suite.

• This is also helpful to mock the "FileSystem" object and "spy" on the end products of AMF's uri-resolution mechanism. This is especially useful when debugging APIKIT Api Sync error cases.

Concerns

• The only concern I have is regarding performance between the JvmFileSystem implementations and the legacy file reading mechanism. Unfortunately I don't see a way to easily benchmark this as disk I/O will affect the measurement a lot.

Note: this small refactor is made to be fully backwards compatible with the previous implementation.