Add support for downloading a specific part/path of a pipeline artifact

**Related command**
```
$ az pipelines runs artifact download
```

**Is your feature request related to a problem? Please describe.**
Let's consider an artifact published under a pipeline. The artifact, say `Foo`, contains multiple versions of `Foo` which are targeted to different OS flavors. I'm trying to download a specific platform version of `Foo`, say `Foo-windows-x64`, from the pipeline using the aforementioned command. But there's no way for me to do this since the only argument the command seems to accept is the artifact name (`Foo` in this case). I'd like to avoid downloading the whole thing since it's large (~500MB vs 87MB for the actual portion I need).

**Describe the solution you'd like**
I can download just the relevant part on the ADO portal. See image below.

So I assume this is technically feasible and just a matter of adding another argument to the above command which would specify a particular path I want to download. e.g.,
```
$ az pipelines runs artifact download --org my-org --project my-proj --run-id 1234 --artifact-name `Foo` --sub-path ".\Foo-windows-x64"
```

**Describe alternatives you've considered**

**Additional context**