feat: add WRITE_TRUNCATE_DATA as an enum value for write disposition (#3752)

* feat: add WRITE_TRUNCATE_DATA as an enum value for write disposition

For existing tables, WRITE_TRUNCATE_DATA preserves schema/constraints
and replaces data.

internal issue: b/406848221

* formatting

Author: shollyman

google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/JobInfo.java

@@ -66,6 +66,12 @@ public enum WriteDisposition {
     /** Configures the job to overwrite the table data if table already exists. */
     WRITE_TRUNCATE,
 
+    /**
+     * Configures the job to retain schema and constraints on an existing table, and truncate and
+     * replace data.
+     */
+    WRITE_TRUNCATE_DATA,
+
     /** Configures the job to append data to the table if it already exists. */
     WRITE_APPEND,