Hiero Solo Action: JSON-RPC Relay Support

Added an option to the Hiero Solo GitHub Action to install a JSON-RPC relay when spinning up a local Hedera network in CI.

The Hiero Solo Action starts a local Hedera network inside GitHub Actions for testing. I added an installRelay input that also installs the JSON-RPC relay, so projects can test Ethereum-compatible tooling against the network.

During review I removed the separate setup.sh script and moved the relay and mirror node installs into conditional steps in action.yml. I tested both the default and the relay-enabled path, and updated the README.

Links: PR #8 • Issue #2