[Bridge] Add support for configure the bridge with multiple substrate URLs for redundancy

[sameh-farouk]

At the moment the TFChain<>Stellar bridge can be configured with one substrate URL

tfchain/bridge/tfchain_bridge/pkg/config.go

Lines 5 to 11 in 77e684b

	type BridgeConfig struct {
	TfchainURL string
	TfchainSeed string
	RescanBridgeAccount bool
	PersistencyFile string
	StellarConfig
	}

Since we are utilizing multiple addresses for Substrate RPC nodes, the bridge should be modified to take full advantage of the redundancy provided by the additional URLs. The TFChain Go client supports multiple URLs, so we should leverage this capability to enhance the reliability and fault tolerance of the bridge.