Consider having "dummy" creds for fake GCS server in local development

[eecavanna]

Currently, the bucket stuff is implemented such that real Google-generated credentials are required.

See developer docs: https://github.com/microbiomedata/nmdc-server/blob/main/docs/development.md#google-cloud-storage

Also, see this issue: fsouza/fake-gcs-server#952 (idea: generate a real file, then "revoke" it on GCS, then store it in the repo and share it with team members — security question/concern: can these files be "un-revoked"?)

CC: @pkalita-lbl, @naglepuff, @eecavanna