[BuildStream] bst artifact show

Hey everyone,

As part of a system that we are building, we would like to be able to

validate that an element can be pulled from a remote cache.

Therefore I would like to propose a new `bst artifact` subcommand:

`bst artifact show`.

This command should be able to show the state of an artifact on cache servers.

The reason why I am proposing a new command and not an addition to

`bst show`, is that since this would require network access, and a full

scheduler to be spawned (because of GRPC), it would be easier to have

it completely separated.

I would envision it like that:

bst artifact show [--limit cache_server...] [ARTIFACTS]

- `--limit` would allow to limit the cache servers in

which to look for the element(s)

The command would behave similarily to other artifacts commands for the

ARTIFACTS value (see `bst artifact log --help` for example).

While our use case is about knowing if something is cached in a particular

server, we might want a command that is more generic than that and that would

help other use cases.

- So, do people have other use cases that a 'bst artifact show' would solve?

- Are we ok with introducing such a new command?

Thanks a lot,

Benjamin