Feature Request: Export user-secrets keys as appsettings.template.json for safer environment config sharing

### Summary

I’d like to propose a new feature for the .NET CLI to help developers sync and document required user secrets without exposing sensitive values. Specifically, the feature would allow exporting **only the keys** of secrets stored via `dotnet user-secrets` to a file like `appsettings.template.json`.

---

### Problem

In collaborative or distributed environments, it's common to use `dotnet user-secrets` for storing sensitive config values locally. However:

- There is no native way to export just the **keys** of secrets to inform other team members or CI/CD pipelines about what environment variables or secrets they need to provide.
- Developers often forget what keys they had configured, especially when returning to the project after some time.
- Manual synchronization between user-secrets and `appsettings.json` (or a `.env`-style placeholder file) becomes error-prone.

---

### Proposed Solution

Add a new CLI command (or a flag) to export just the secret keys from the current project’s user-secrets:

```bash
dotnet user-secrets export --keys-only > appsettings.template.json

```

Or as a separate command:
```bash
dotnet user-secrets sync-template
```

This would generate an output like:
```json
{
"ConnectionStrings": {
"Default": ""
},
"Jwt": {
"Issuer": "",
"Key": ""
}
}
```

🔒 Keeps secrets safe (no values exposed)

📦 Provides a standardized, sharable template

👥 Improves team collaboration and onboarding

🚀 Helps CI/CD environments understand required config keys

✅ Mirrors common .env.example practice found in Node.js and other ecosystems

Alternatives Considered
Manually documenting keys in README

Using a fake appsettings.Development.json with dummy values (risky if accidentally committed)

Writing custom scripts to parse secrets.json