Add support for using GCE assigned service accounts

[andyfoston]

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Add support for using Google GCE service accounts that are assigned to a VM, rather than using an explicit service account key.

Using a static key is a problem for my organisation as these need to be frequently rotated. However, when a service account is assigned to a GCE Virtual Machine, a short lived key is made available via the GCE metadata API so this completely removes this issue for us.

The gcloud-aio GCS client library supports using this, so I've created a PR to add support for this. To use this feature, the key_file can be omitted from the medusa.ini file.

I have tested this with a Google Compute Engine VM, and this works as expected. I think this will also work when used with GKE Workload Identity too (for K8ssandra), but I haven't tested this (related to #558)

┆Issue is synchronized with this Jira Story by Unito
┆Issue Number: MED-115