Add dev shell environment

[Ifropc]

What

This PR adds a devshell environment using nix

Why

Currently, in order to develop stellar-cli locally you need to install some dependencies, and sometimes you also need to modify your .<shell>rc file to make your environment easy to use for development.
However, this makes switching between different branches/versions or switching between development/production builds more complicated.
This change adds a configuration for a development shell, which automatically does the following:

1. Installs all necessary dependencies (only accessible in the shell) -> rust, make, libudev, etc. without affecting your global environment. Dependencies are pinned into the lockfile. We can always update rust version to latest stable or change it to nightly with re-generating lockfile/changing nix config. Dependencies are installed once and cached. Can clean them out of your system with nix collect-garbage.
2. Setup stellar alias, configure development key and environment (set STELLAR_NETWORK and STELLAR_ACCOUNT variables)
3. Add up-to-date autocomplete in the shell

Developer simply needs to install nix and run nix develop

This should work both on any Linux distro, Mac (Darwin builds are available for all packages in nixpkgs) and Windows WSL

Known limitations

nix is required to be installed in the system, with v2.20+
Currently bash is used as a default shell. It's possible to use other shells with nix develop -c zsh

[Ifropc]

Examples:

# No rust toolchain is installed in my system, and I have `stellar` installed in ~/.cargo/bin/stellar
ifro@nixos:~/projects/stellar-cli/ > which cargo
cargo not found
ifro@nixos:~/projects/stellar-cli/ > stellar --version
stellar 21.2.0 (v20.0.0-248-g96e819c949089a6744621164bb406fc411de676e-dirty)
soroban-env 21.2.0 (8809852dcf8489f99407a5ceac12625ee3d14693)
soroban-env interface version 90194313216
stellar-xdr 21.2.0 (9bea881f2057e412fdbb98875841626bf77b4b88)
xdr curr (70180d5e8d9caee9e8645ed8a38c36a8cf403cd9)
ifro@nixos:~/projects/stellar-cli/ > nix develop
Using nix (Nix) 2.20.8
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.17s
     Running `target/debug/stellar keys add dev_key`
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.17s
     Running `target/debug/stellar completion --shell bash`

# Have rust toolchain in devshell, and `stellar` points to the freshly built binary 
[ifro@nixos:~/projects/stellar-cli]$ which cargo
/nix/store/i223gfb0a86hxywdsqxmyy291z511056-rust-default-1.80.0/bin/cargo

[ifro@nixos:~/projects/stellar-cli]$ stellar --version
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.17s
     Running `target/debug/stellar --version`
stellar 21.3.0 (v20.0.0-257-g0e615b6c61d03ab294fd6ccab42b1b08f317a437)
soroban-env 21.2.0 (8809852dcf8489f99407a5ceac12625ee3d14693)
soroban-env interface version 90194313216
stellar-xdr 21.2.0 (9bea881f2057e412fdbb98875841626bf77b4b88)
xdr curr (70180d5e8d9caee9e8645ed8a38c36a8cf403cd9)

[Ifropc]

Also no documentation on it at the moment, open to suggestions on where to add it (README.md or CONTRIBUTING.md)

[leighmcculloch]

Documentation should be in CONTRIBUTING.md I think.

[leighmcculloch]

I don't know if many folks deving on the cli use nixos, but sounds like you do @Ifropc. Can you remove the secret key and setup assumptions so that the flake is only the tool setup and doesn't make other assumptions? Otherwise lgtm.

[Ifropc]

I added secret key just cause I thought it'd be a good idea to easily test things locally (so shared dev secret key is fine) but I now I think it's better if i remove exporting the variable when key is not defined instead
Also you don't need to use nixos for that! Nix is just a package manager and environment configuration tool so you can install and use it in any Linux distro or without impacting your native package manager (as all installed are done against your dev shell)

[leighmcculloch]

I added secret key just cause I thought it'd be a good idea to easily test things locally (so shared dev secret key is fine) but I now I think it's better if i remove exporting the variable when key is not defined instead

It's very easy for devs to create a key, it's a single command, stellar keys generate me.

[TomMD]

Before I open an issue perhaps asking here makes sense: why the libudev0 dependency? That means it can't be used on mac/darwin.

[Ifropc]

Hey @TomMD answered in #1661 thanks for raising this up