Make success hook public

As described in #125 , the original
idea was to be able to write
final Task<TableId> task = Task.named("task")
    .ofType(TableId.class)
    .context(BigQueryContext.create(table))
    .context(BigQueryOperator.create())
    .process((stagingTable, bq) -> bq.job(
        JobInfo.of(QueryJobConfiguration.newBuilder("SELECT foo FROM input")
            .setDestinationTable(stagingTable.tableId())
            .build()))
        .success(response -> stagingTable.publish()));

But because success is package private this use case is only allowed in
the classes in the same package, like the unit test

contrib/flo-bigquery/src/main/java/com/spotify/flo/contrib/bigquery/BigQueryOperation.java

@@ -52,7 +52,7 @@ BigQueryOperation<T> job(Fn<JobInfo> jobRequest) {
   /**
    * Specify some action to take on success. E.g. publishing a staging table.
    */
-  BigQueryOperation<T> success(F1<JobInfo, T> success) {
+  public BigQueryOperation<T> success(F1<JobInfo, T> success) {
     this.success = Objects.requireNonNull(success);
     return this;
   }