Proxies a JSON-RPC 2.0 request to the Fusion L2 OP-Geth node.
This route is for the Fusion L2 only. Only Fusion L2 methods are supported —
JSON-RPC for any other connected chain must go to the Fusion Proxy's multi-DLT
endpoint /dlts/{dlt}/networks/{networkId}/rpc/{apiKey} (see the fusion-proxy spec).

Authentication is accepted via the Overledger-issued API key as the
{apiKey} path segment, the X-API-Key header, or an
Authorization: Bearer <jwt> Keycloak token. The X-API-Key header
takes precedence over the others when present.