`bindle push` needs to sign using the `creator` role for the current user's key

Some flags might be necessary to override defaults, but here is what I am thinking as the default behavior:

User runs bindle push
Client loads user's signing secret key, failing if the key is not found
Client loads the invoice.toml
Client signs the invoice object (support is in signature.rs)
Client appends signature as a [[signature]] block on the invoice.toml
- Option A: The invoice with the signature is stored on disk
- Option B: This version of the invoice is in memory only, since there is little value in keeping the signature on the artifact on disk
- Client pushes the invoice to Server

deislabs / bindle