Add brev exec command for remote script and command execution

[jinendrajj]

Add brev exec Command for Remote Workspace Execution
This PR introduces a new brev exec command that enables users to execute scripts or commands on remote workspaces without manually setting up SSH connections.

Features

• Script Execution: Upload and run local shell scripts on workspaces
• Direct Commands: Execute single commands remotely
• Async Mode: Fire-and-forget execution with --async flag
• Custom Working Directory: Specify execution directory with --dir
• Robust Connection Handling: Automatic retries for SSH and SCP operations
• Progress Indicators: Visual feedback with spinners during connection setup

Usage

brev exec <workspace> --script ./setup.sh
brev exec <workspace> --command "ls -la"
brev exec <workspace> --script ./deploy.sh --async
brev exec <workspace> --command "npm start" --dir /app

Implementation

• Automatic SSH connection management with retry logic
• Secure file transfer via SCP with auto-cleanup
• Real-time output streaming for synchronous execution