auth: allow to pass credentials as text

[orsinium]

1. Allow passing credentials as a pathlib.Path object.
2. Allow passing credentials as text content instead of a path to a file.
3. Propagate exceptions for an invalid JSON if it is explicitly passed, not only for IO errors. And all types of exceptions, really, because why not.
4. Allow CLOUDSDK_CONFIG to be a path to the directory, not only to the file.
5. Cover it all with tests.

[orsinium]

Wow. Why there are tests on CI for Python 2.7 if the minimal supported version by the package is 3.6? I see zero downloads from Python 2.7 ever. Am I missing something? 🤔

[TheKevJames]

@orsinium ah, yes, the Py2 compatibility here is certainly odd. As mentioned in the readme, this repo actually contains the codebase for two projects: both gcloud-aio-* (which uses asyncio and is thus 3.6+) and gcloud-rest-* (which is py2.7-compatible). You can see there are a few py2 downloads for that project (https://pypistats.org/packages/gcloud-rest-auth); we'll eventually drop support, but for now we have a couple legacy users we're still supporting.

The gcloud-rest-* version of the libraries can be debugged by grabbing 'em from CI here (https://app.circleci.com/pipelines/github/talkiq/gcloud-aio/1693/workflows/021d167d-94c7-4825-a4d9-f8a6b194e50a/jobs/34866/artifacts). The CI job for generating them could also theoretically be run locally, but it's unfortunately complex (https://github.com/talkiq/gcloud-aio/blob/26a34a3a2ed1e22622235e16ee5c9c35a3e745c3/.circleci/config.yml#L142).

I will review this code as soon as I can -- previously, this logic matched behaviour with the google-auth library, so I'll need to make sure all the same cases are covered and any differences are documented clearly.