Add hierarchical credentials for S3 based storages

[Emre Sahin]

What I mean is something like:

For example, for S3,

• If there is an environment variable XVC_STORAGE_ACCESS_KEY_ID_remote1, it's considered to have more priority over other credentials.
• Q: Should we continue to support XVC_STORAGE_ACCESS_KEY_ID (without remote1 per storage)? I think for multiple storage types, this can cause difficulties. If I want to access GCS with its credentials and S3 with its credentials, having a single environment variable makes everything complex. OTOH if there is such a single remote storage, that might work. But for single storage, the above scheme might also work. There is no need for a common credentials scheme for all storage types. If user wants to set credentials for remote1 specifically, they can do using the above environment variable.

[Emre Sahin]

The following variables are recognized by Xvc for credentials

AWS_ACCESS_KEY_ID
AWS_SECRET_ACCESS_KEY

DIGITAL_OCEAN_ACCESS_KEY_ID
DIGITAL_OCEAN_SECRET_ACCESS_KEY

WASABI_ACCESS_KEY_ID
WASABI_SECRET_ACCESS_KEY

GCS_ACCESS_KEY_ID
GCS_SECRET_ACCESS_KEY

R2_ACCESS_KEY_ID
R2_SECRET_ACCESS_KEY

MINIO_ACCESS_KEY_ID
MINIO_SECRET_ACCESS_KEY