refactor: add interface DatabendConnection.

Author: youngsofun

README.md

@@ -159,7 +159,7 @@ The following example demonstrates how to upload data and load it into a table:
 ```java
 // 1. Upload a file to the internal stage
 Connection conn = DriverManager.getConnection("jdbc:databend://localhost:8000");
-FileTransferAPI api = conn.unwrap(DatabendConnection.class);
+DatabendConnection api = conn.unwrap(DatabendConnection.class);
 
 FileInputStream fileStream = new FileInputStream("data.csv");
 api.uploadStream(

databend-jdbc/src/main/java/com/databend/jdbc/Capability.java

@@ -2,7 +2,7 @@
 
 import com.vdurmont.semver4j.Semver;
 
-public class Capability {
+class Capability {
     private final boolean streamingLoad;
     private final boolean heartbeat;
     public Capability(Semver ver) {

databend-jdbc/src/main/java/com/databend/jdbc/ConnectionProperties.java

@@ -9,7 +9,7 @@
 import java.util.Set;
 
 // all possible JDBC properties options currently supported by databend driver
-public final class ConnectionProperties {
+final class ConnectionProperties {
     public static final ConnectionProperty<String> USER = new User();
     public static final ConnectionProperty<String> PASSWORD = new Password();
     public static final ConnectionProperty<Boolean> SSL = new Ssl();

databend-jdbc/src/main/java/com/databend/jdbc/DatabendClientLoadBalancingPolicy.java

@@ -3,7 +3,7 @@
 import java.net.URI;
 import java.util.List;
 
-public class DatabendClientLoadBalancingPolicy {
+class DatabendClientLoadBalancingPolicy {
     static class DisabledPolicy extends DatabendClientLoadBalancingPolicy {
         @Override
         public String toString() {

databend-jdbc/src/main/java/com/databend/jdbc/DatabendColumnInfo.java

@@ -10,7 +10,7 @@
 
 import static java.util.Objects.requireNonNull;
 
-public class DatabendColumnInfo {
+class DatabendColumnInfo {
     private static final int VARBINARY_MAX = 1024 * 1024 * 1024;
     // current longest time zone is 32
     private static final int TIME_ZONE_MAX = 40;