JSON-RPC endpoint for ARC methods

Single JSON-RPC 2.0 POST endpoint. Methods currently implemented under the arc namespace:

1. arc_getCertificate(height: u64) -> RpcCommitCertificate Returns the consensus commit certificate for the specified height if available. Error Mapping (error.code -> meaning): -32602 Invalid params (height < 1) -32004 Not found (no certificate upstream) -32005 Upstream unreachable (connect errors) -32603 Internal (timeout / decode / generic upstream HTTP status) Curl Example: curl -s -X POST -H 'Content-Type: application/json'
   --data '{"jsonrpc":"2.0","method":"arc_getCertificate","params":[7],"id":1}'
   http://127.0.0.1:8545

2. arc_getVersion() -> RpcVersionInfo Returns version/build information for the node execution layer. Fields: git_version: Git tag or short commit hash git_commit: Full git commit hash git_short_hash: Short git commit hash cargo_version: Cargo package version Curl Example: curl -s -X POST -H 'Content-Type: application/json'
   --data '{"jsonrpc":"2.0","method":"arc_getVersion","params":[],"id":1}'
   http://127.0.0.1:8545

NOTE: This path '/' is shared by all JSON-RPC methods; separate request/response schemas are provided for clarity.