chore: make DatabaseClient accessible in the Connection API (#1847)

Makes the underlying `DatabaseClient` accessible in the Connection API so this can be used by drivers that need direct access to it.

See https://github.com/GoogleCloudPlatform/pgadapter/blob/8f0d47785e04dc9db6ee275074777849f006d797/src/main/java/com/google/cloud/spanner/pgadapter/utils/MutationWriter.java#L439

Author: olavloite

google-cloud-spanner/clirr-ignored-differences.xml

@@ -60,10 +60,14 @@
     <className>com/google/cloud/spanner/spi/v1/SpannerRpc</className>
     <method>com.google.api.gax.longrunning.OperationFuture copyBackup(com.google.cloud.spanner.BackupId, com.google.cloud.spanner.Backup)</method>
   </difference>
+  <difference>
+    <differenceType>7012</differenceType>
+    <className>com/google/cloud/spanner/connection/Connection</className>
+    <method>com.google.cloud.spanner.DatabaseClient getDatabaseClient()</method>
+  </difference>
   <difference>
     <differenceType>7012</differenceType>
     <className>com/google/cloud/spanner/DatabaseAdminClient</className>
     <method>com.google.api.gax.longrunning.OperationFuture createDatabase(java.lang.String, java.lang.String, com.google.cloud.spanner.Dialect, java.lang.Iterable)</method>
   </difference>
-
 </differences>

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/Connection.java

@@ -23,6 +23,7 @@
 import com.google.cloud.spanner.AbortedException;
 import com.google.cloud.spanner.AsyncResultSet;
 import com.google.cloud.spanner.CommitResponse;
+import com.google.cloud.spanner.DatabaseClient;
 import com.google.cloud.spanner.Dialect;
 import com.google.cloud.spanner.ErrorCode;
 import com.google.cloud.spanner.Mutation;
@@ -1104,6 +1105,12 @@ default Dialect getDialect() {
     throw new UnsupportedOperationException("Not implemented");
   }
 
+  /** The {@link DatabaseClient} that is used by this {@link Connection}. */
+  @InternalApi
+  default DatabaseClient getDatabaseClient() {
+    throw new UnsupportedOperationException("Not implemented");
+  }
+
   /**
    * This query option is used internally to indicate that a query is executed by the library itself
    * to fetch metadata. These queries are specifically allowed to be executed even when a DDL batch

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/ConnectionImpl.java

@@ -332,6 +332,11 @@ public Dialect getDialect() {
     return dbClient.getDialect();
   }
 
+  @Override
+  public DatabaseClient getDatabaseClient() {
+    return dbClient;
+  }
+
   @Override
   public boolean isClosed() {
     return closed;