A sub-command to generate test artefacts

[andrewpatto]

Add a sub-command that can be used to make example files/objects.

For instance, cloud-checksum artefact --bucket blah --prefix samples/ABC "uneven-part-size-sample-large"

Some of our testing can be mocked - but some is going to rely on creating actual S3 objects with various difficult settings. We will almost have to hand roll their creation - as for instance aws s3 cp cannot create objects with differing part sizes.

It will not be a lot of code, just lots of boilerplate AWS S3 PutObjects with some minor differences.

I have some versions of this written in Typescript - consider just converting over to Rust.