CLI pubkey optarg

[jmwample]

Through misunderstanding commit c04f28f removed the ability to specify a custom public key by changing the file assets/station_pubkey. re-adding this ability and adding a command line optarg to specify a pubkey file for the cli testing utility would be helpful

Use the assets.go API, namely the SetPubkey function or some wrapper of it.

• Re-Enable pubkey overrides using assets/station_pubkey by undoing c04f28f
• Add CLI optarg to cli/main.go
• Document the process to use this.