Separate metadata folder for testnet

Create a separate folder for the testnet
- New folder `providers-testnet`
- Update the CI to use this new folder for the testnet deployment.
- Keep the [_example.json](https://github.com/AztecProtocol/staking-dashboard/blob/main/providers/_example.json) and [README.md](https://github.com/AztecProtocol/staking-dashboard/blob/main/providers/README.md) files in the new `providers-testnet` folder.