Implement the "source" side of profile parsing (#594)

* Introduce environment abstraction

* Update region provider to use ProcessEnvironment

* Update credentials provider to use env var abstraction

* implement the "source" side of profile parsing

This commit adds support for the "source" side of profile file parsing:
Given a filesystem/environment, what files should we actually be reading?

* run runtime tests on windows

* Update changelog

* Update ci.yaml

don't run rustfmt on windows

* Apply suggestions from code review

Co-authored-by: John DiSanti <jdisanti@amazon.com>

* cr feedback / refactorings to profile source loading

* refactor runtime test job

* Add missing workspace cargo.toml

* Fix working directory issue

* fix windows test for new API

* special case test for windows behavior

* Always compile the tests but conditionally ignore

* Apply suggestions from code review

Co-authored-by: John DiSanti <jdisanti@amazon.com>

Co-authored-by: John DiSanti <jdisanti@amazon.com>