Support multiple transmission protocols

[clcc2019]

Describe the problem/challenge you have

Velero currently supports a limited set of storage backends for Kopia, and there is a need for broader support for additional storage protocols such as SFTP, rclone, and Google Cloud Storage (GCS). The lack of these options restricts flexibility for users who want to utilize Velero in environments with these storage protocols.

Describe the solution you'd like

I would like Velero to support additional Kopia storage protocols, specifically:

SFTP: Allow backup and restore operations to be performed using SFTP as the storage backend.
rclone: Add support for rclone-compatible storage backends, enabling users to integrate with various cloud providers and protocols.
Google Cloud Storage (GCS): Provide native support for GCS as a backup target via Kopia, allowing for seamless cloud integration.
Anything else you would like to add:

Supporting these additional protocols will significantly improve Velero’s versatility and usability in diverse environments. These enhancements could be beneficial for users with specific cloud storage needs or those operating in restricted environments where these protocols are preferred.

Environment:

Velero version (use velero version):1.15.2
Kubernetes version (use kubectl version): v1.31.6
Kubernetes installer & version:
Cloud provider or hardware configuration:
OS (e.g. from /etc/os-release):linux

[kaovilai]

I think we would want to enable more around the same time #1229 is addressed.

I don't think we ever want to support more kopia backends without considering those similar backends for object store as well.

[kaovilai]

Perhaps if we agree on all backends kopia support then we'd essentially have velero write backup manifests to disk, then kopia to those backends. Rather than replicating each kopia backend for k8s manifests.