feat: support env vars for settings

[hrishikesh-k]

Description

This PR instroduces support for setting environment variables for settings configured in manifest instead off relying on interactive shells. The reason is to support a use-case as follows:

I'm using Vue + Vite to develop a Zendesk App. I have configured some scripts in my package.json as follows:

{
  "scripts": {
    "build": "dotenv -- npm-run-all lint typecheck build:vite build:zd",
    "build:vite": "vite build",
    "build:zd": "zcli apps:package ./dist/",
    "dev": "dotenv -- npm-run-all --parallel \"build:vite -- --emptyOutDir false --watch\" dev:zd",
    "dev:vite": "vite dev",
    "dev:zd": "zcli apps:server ./dist/",
    "lint": "biome check",
    "typecheck": "vue-tsc --noEmit"
  }
}

So far, I was just running npm run dev and it was all working fine for me from a single shell. However, then I introduced a secure setting which now prompts me to enter some text when I run zcli apps:server. Since I'm running this with npm-run-all, I never get a chance to enter my text and the script fails.

Instead, if the value can be set from environment variables, that would solve the issue.

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Checklist

• 💂‍♂️ includes new unit and functional tests