FCM v1: use async version of google-auth and add HTTP proxy support

[MatMaul]

Fix #371.

[MatMaul]

Another kick is required to trigger the workflows, this is annoying. @devonh if you can trigger them again please 🙏 .

[devonh]

I tested this out locally and it does appear to fix the proxy issue.
I'm just double checking with the team whether we are okay with using the experimental aiohttp support from the google-auth library.

Otherwise there are just a few small changes requested.

[MatMaul]

Thanks for the fast review.

I've address the comments and also differentiate the error cases for service_account_file between not specified and invalid file, so we can surface the underlying load error.

[MatMaul]

Also perhaps an useful info, this is deployed in our preprod (with an HTTP proxy) with no trouble.

[devonh]

This is looking good.

Would you be able to add a couple of unit tests?
Particularly around loading the service account file and _get_auth_header(). (patching out the call to credentials.refresh)

Once those are in place I am comfortable with merging this.

[MatMaul]

I've added a fake service account file to test loading, and create a FakeCredentials to exercise _get_auth_header.

Unfortunately refresh is never called and is blocking, for some async magic I don't really understand between Twisted and asyncio I believe.
I am a bit stuck so I pushed the changes in case someone has an idea.

[devonh]

I've added a fake service account file to test loading, and create a FakeCredentials to exercise _get_auth_header.

Unfortunately refresh is never called and is blocking, for some async magic I don't really understand between Twisted and asyncio I believe. I am a bit stuck so I pushed the changes in case someone has an idea.

Thank you for adding the tests.
I'll take a look at the blocking issue today to see if I can figure anything out with that.