add more predefined value for epoch number

src/main/java/conflux/web3j/request/Epoch.java

@@ -21,6 +21,14 @@ static Epoch latestMined() {
 		return DefaultEpoch.LATEST_MINED;
 	}
 
+	static Epoch latestCheckpoint() {
+		return DefaultEpoch.LATEST_CHECKPOINT;
+	}
+
+	static Epoch latestConfirmed() {
+		return DefaultEpoch.LATEST_CONFIRMED;
+	}
+
 	static Epoch numberOf(long number) {
 		String value = Numeric.encodeQuantity(BigInteger.valueOf(number));
 		return new EpochByValue(value);
@@ -39,7 +47,9 @@ static Epoch hashOf(String blockHash) {
 enum DefaultEpoch implements Epoch {
 	EARLIEST("earliest"),
 	LATEST_STATE("latest_state"),
-	LATEST_MINED("latest_mined");
+	LATEST_MINED("latest_mined"),
+	LATEST_CHECKPOINT("latest_checkpoint"),
+	LATEST_CONFIRMED("latest_confirmed");
 
 	private String value;