Extract rad remote helper into lib

[rudolfs]

For the project checkout feature we'll need to package the rad-remote-helper together with Upstream.
For this I looked into the following options:

1. Forcing Upstream to build the git-helpers binary from its radicle-link dependency, so far seems to be impossible in Cargo:

   • Cargo needs a way to run an executable out of a build-dependency rust-lang/cargo#872
   • Allow dependencies to build binaries rust-lang/cargo#748

2. Change Upstream proxy into a top-level Rust workspace and include radicle-link as a sub-component: not possible because radicle-link is already organized as a workspace and nested workspaces are not allowed:

   • https://github.com/nox/rust-rfcs/blob/master/text/1525-cargo-workspace.md#validating-a-workspace
   • https://stackoverflow.com/questions/49849878/how-to-deal-with-multiple-nested-workspace-roots

3. Make upstream build and package scripts do a cargo install --git https://github.com/radicle-dev/radicle-link.git --rev 269a7ededafd7373ebfb03210770d0b3778002f: blows up build times both for devs and on CI because build artefacts are not shared between proxy and the external cargo install.

4. Extracting the git-remote-rad functionality into a lib, so we can create a separate binary within Upstream that will be packaged along with proxy.

This PR implements option nr. 4.

[FintanH]

Makes sense to me at least 👍

[cloudhead]

Instead of putting this code here, I'd suggest:

• Adding pub mod remote_helper here
• Creating a remote_helper.rs file and moving this code into that file
• Renaming the run_rad_remote_helper function to run
• Invoking the function from main with remote_helper::run()

[rudolfs]

1547c52

[FintanH]

Good shout @cloudhead, this looks better organised now 🙌

[cloudhead]

Nice 👍