RPC: Add getFeeCalculatorForBlockhash method call (#8687)

Returns the `FeeCalculator` associated with the given blockhash, or
`null` if said blockhash has expired

client/src/rpc_response.rs

@@ -152,6 +152,12 @@ pub struct RpcBlockhashFeeCalculator {
     pub fee_calculator: FeeCalculator,
 }
 
+#[derive(Serialize, Deserialize, Clone, Debug)]
+#[serde(rename_all = "camelCase")]
+pub struct RpcFeeCalculator {
+    pub fee_calculator: FeeCalculator,
+}
+
 #[derive(Serialize, Deserialize, Clone, Debug)]
 #[serde(rename_all = "camelCase")]
 pub struct RpcFeeRateGovernor {