Make success hook public

[narape]

Hey, I just made a Pull Request!

Description

Make BigQueryOperation.success public

Motivation and Context

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 in the unit test

Have you tested this? If so, how?

Same test suit. Manually I also created a dummy class with main and copied the code above

Checklist for PR author(s)

• Changes are covered by unit test
• [x ] All tests pass
• Code coverage check passes
• Error handling is tested
• Errors are handled at the appropriate layer
• Errors that cannot be handled where they occur are propagated
• Relevant documentation updated
• This PR has NO breaking change to public API
• This PR has breaking change to public API and it is documented

Checklist for PR reviewer(s)

• This PR has been incorporated in release note for the coming version
• Risky changes introduced by this PR have been all considered