Store api key in keychain or credential manager instead of plaintext

## Overview

Per a discussion with @rseldner we noticed ecctl stores API key in plaintext.
This is insecure.

## Possible Implementation

We would propose to store key in keychain (macOS) or credential manager (WinOS), etc.
If hard, then avoid saving it in `$HOME/.ecctl/config.json`, but let user specify it in env variables or bash_profiles, to reduce risk.

## Testing

n/a

## Context

given api key is extremely important to ops the deployment, we should keep it safe and secure

## Your Environment

macOS