feat: Add cancelQuery() to PGConnection public interface

Adds cancelQuery() to the extended public interface PGConnection to allow
for issueing of cancels directly to connections rather than having to keep
track of individual statements. The internals of the driver and the backend
server already operate at a connection level (not a statement level) and
this patch only exposes the existing cancelQuery() method publicly.

pgjdbc/src/main/java/org/postgresql/PGConnection.java

@@ -182,6 +182,11 @@ public interface PGConnection {
    */
   int getBackendPID();
 
+  /**
+   * Sends a query cancellation for this connection.
+   */
+  void cancelQuery() throws SQLException;
+
   /**
    * Return the given string suitably quoted to be used as an identifier in an SQL statement string.
    * Quotes are added only if necessary (i.e., if the string contains non-identifier characters or