v3: env var management API

[ericallam]

✅ Checklist

• Create env var (in a specific env)
• Read env var (in a specific env)
• Edit env var (in a specific env)
• List env vars (in a specific env)
• Delete env var (in a specific env, but also completely deletes the env var if last env value)
• Add to SDK
• Add documentation

[changeset-bot]

⚠️ No Changeset found

Latest commit: 3ea4de7

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