Feature/PR proposal: s3 target support about virtnbdbackup

[abbbi]

thanks for the feedback! Ive thought about an s3 backend implementation but personally i don't use s3 as storage
for backups (or lets say, s3 compatible ones like minio) because i prefer to keep my data locally. The main problem is
that its hard for me to have such a feature and maintain its codebase if i don't have any usage for it.

Also, i would have opted for s3fs implementions first, but i dont know about their stability as you mention.

As for a PR, i dont know if i would accept, probably only if i want to use the feature myself and then it needs to get tested in the automatic CI tests to ensure its functionality.

[abbbi]

One idea might be to redesign the output writer stuff to be "plugin" capable, and use regular URI based
notation:

 -o fs://dir / -o dir/-> loads module output/dir/
 -o - > loads module output/stdout
 -o s3://user@foo -> loads module output/s3

so the s3 implementation could be shipped as sort of a additional "plugin" maintained by the persons caring about
it.

[nward]

Hah, that's exactly what I was thinking about when pondering your reply. I think that the same would be possible for the encryption. Perhaps both encryption and compression can be "filters" in a chain, or something like that.