Simplify usage with gateway-proxy

[Cafe137]

The workaround is to use --stamp 0000000000000000000000000000000000000000000000000000000000000000 to get around the hard requirement of passing stamp for all upload actions

We could add --no-stamp but that would confuse things.

Maybe we could plan out a service discovery mechanism, e.g. the CLI does a preflight request to check for automatic stamp management at the remote endpoint, and lift the stamp requirement in that case.