v3: env var management API

[ericallam]

✅ Checklist

• [x] Create env var (in a specific env)
• [x] Read env var (in a specific env)
• [x] Edit env var (in a specific env)
• [x] List env vars (in a specific env)
• [x] 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[bot]]

⚠️ No Changeset found

Latest commit: 3ea4de7f699a176863023e86d813d79620c91bad

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