Allow specifying GCS credentials directly through the environment

[echozio]

Is your feature request related to a problem? Please describe.
When using the operator there's seemingly no good way to add credentials for GCS with AUTH=implicit. Since it doesn't allow us to add volumes or volumeMounts and GOOGLE_APPLICATION_CREDENTIALS is expected to point to a file.

Describe the solution you'd like
An alternative environment variable where we can include the whole service account key json encoded in base64. E.g. GOOGLE_APPLICATION_CREDENTIALS_BASE64

Describe alternatives you've considered

• Using GCS with the S3 driver and HMAC keys instead, as these can be specified in their entirety from the environment.
• Shut down the cluster, add the credentials to each volume and read them from there.

Additional context
This could also be solved by implementing support for either additional volumes or additional initContainers in the operator, but not having to rely on the filesystem at all for this seems like a better approach.

Jira issue: CRDB-39766

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