add proxy support to the package download subcommand

[maminrayej]

In continuation with #4624, this PR adds proxy support to the wasmer package download subcommand.

[syrusakbary]

Address comment

[ayys]

Potential future changes: we will probably need to do the same for wasmer run (I think that uses a different code path to download the package).
Also, the code to upload file to google storage would also break I suppose? so we will need to add proxy there as well. (cc: @xdoardo since he is working on the publish stuff)

[maminrayej]

Potential future changes: we will probably need to do the same for wasmer run (I think that uses a different code path to download the package).

The code path for wasmer run was covered in #4624 .

Also, the code to upload file to google storage would also break I suppose? so we will need to add proxy there as well

That's a good point. I can check that code path as well.

[syrusakbary]

Approved once comment is addressed