Allow GCS client to use credentials from the environment if needed

[cmaster11]

When running in GCP environment, the credentials can be fetched automatically by the GCP SDK, and there is no need to specify any additional JSON/env variables (https://cloud.google.com/docs/authentication/production#automatically).

[Xuanwo]

diff check failed, use make build can resolve this problem.

[Xuanwo]

We need to support both env and other ways.

How about using google.FindDefaultCredentials instead?

ref: https://pkg.go.dev/golang.org/x/oauth2/google#FindDefaultCredentials

[cmaster11]

FindDefaultCredentials gets called automatically by the GCS client if no credentials are provided (e.g. if you don't use the WithCredentialsJSON).

So, if you pass env as credential type, the client will just fetch them all at runtime.

This happens here: https://github.com/googleapis/google-api-go-client/blob/master/internal/creds.go#L50

[Xuanwo]

gets called automatically by the GCS client if no credentials are provided

Yep, I know about this logic here.

But I don't want to depend on this behavior. I prefer to control them by ourselves.

• If the user uses base64 or file, it's OK, read them and use WithCredentialsJSON to construct the client.
• If the user uses env, we use FindDefaultCredentials to read default credentials from the env.

Everything is clear, no need to read the code provided by deps.

What do you think?

[cmaster11]

Absolutely got your point :) I cleaned up the code

[cmaster11]

diff check failed, use make build can resolve this problem.

Ran and diff is fine now :)

[Xuanwo]

Thanks for your contributions!