[monero-project/monero] relay_tx error via monero_wallet_rpc: ‘Failed to parse tx metadata.’ (#4283)

When using `relay_tx` via JSON-RPC (`monero-wallet-rpc,`) error code `-27` (`»Failed to parse tx metadata»`) is thrown. There doesn’t appear to be any way to provide `tx_metadata` to `relay_tx`.

To reproduce, generate a transaction with `do_not_relay`, `get_tx_hex`, and `get_tx_metadata` all set to true. Use `relay_tx` with param `{ hex: tx_blob }`. There is not a clear way to pass `tx_metadata` as a param from looking at `wallet_rpc_server.cpp`.