feat: make ``load_credentials_from_file`` a public method (#530)

* feat: make load_credentials_from_file public and alllow scopes

* test: update tests

* fix: raise error for json with no type

* test: fix test names

* refactor: simplify control flow

* fix: raise coverage

* test: update test

Co-authored-by: arithmetic1728 <58957152+arithmetic1728@users.noreply.github.com>
Co-authored-by: Sijun Liu <sijunliu@google.com>
3 files changed