Show new code layout

[masci]

As discussed internally, this PR has the sole purpose of showing one of the options being considered to refactor the code in order to ease maintenance and readability.

The general philosophy is to consider commands a service provider to be used by the CLI and possibly by other consumers. The commands public API is mapped with Protocol buffers so that can be consumed through a gRPC client as well. These would be the macro components of the codebase after this refactoring:

• package cli: implements the CLI
• package commands: implements high-level functions used by the CLI and exported through rpc
• package arduino: implements low-level functions, used by commands and go-importable in other projects (provided one knows what they're doing)

proto-gen folders contains the (versioned) generated code of the components we export through rpc (for now, only commands).

Implementation-wise, this is the less invasive option, mainly focues on a few things:

• use Task to orchestrate protoc
• move proto definitions close to the models they represent (under commands) so we don't need the root package rpc anymore
• make the use of generated code explicit and versioned (see rpc_v1)
• reduce the number of packages at the root, moving daemon within commands