Feature: Support All Flag for Deployment Deploy Method

[yokeTH]

Background

I was bored when I used deploys.app GitHub action. I cannot set the deploy type, e.g. when I want to deploy a Discord bot, I have to log in to deploys.app and deploy the broken deployments first because I cannot change the deploy type to use the action. This CLI will make it possible for me to add more flags in deploys.app GitHub action.

Change

• Introduced Nix flake support by adding .envrc and flake.nix files, enabling reproducible development environments with Go tooling and dependencies. (It's just a dev dependency. Tell me if you want me to remove it. It’s nice to have for other contributors.)
• Refactored the deployment logic in internal/runner/runner.go by moving the "deploy" subcommand into a new deploymentDeploy function, enabling easier maintenance and extension.
• Implemented input validation for deployment type and protocol, and cron schedule format using the robfig/cron/v3 library.