[improve][txn] Move TransactionCoordinatorClient to public API

[TomaszGaweda]

Related to: #26452

This PR removes of @PrivateApi annotations from TransactionCoordinatorClient and with moves TransactionCoordinatorClient to a public API for PulsarClient.

Fixes #26452

Motivation

One thing that is missing is our ability for commit or abort a transaction by ID itself. It is super useful (mandatory for exactly-once) in streaming engines, like Hazelcast Jet or Apache Flink. We (Hazelcast) track snapshot transaction IDs (including TxnID), in case of failures we want to be able to reliably commit or abort transactions in progress. Right now it's impossible to do officially.

I can do following:

		PulsarClientImpl impl = (PulsarClientImpl) client;
        TransactionCoordinatorClient tcClient = impl.getTcClient();
        if (commit) {
            try {
                tcClient.commit(transactionId.txId);
            } catch (TransactionCoordinatorClientException e) {
                throw new RuntimeException(e);
            }
        } else {
            try {
                tcClient.abort(transactionId.txId);
            } catch (TransactionCoordinatorClientException e) {
                throw new RuntimeException(e);
            }
        }

and Flink was also doing this.

However, it relies on casting to implementation classes classes and using interface marked as PrivateApi.

Modifications

• Removal of PrivateApi annotations
• Adding neccesary method in PulsarClient to access TransactionCoordinatorClient

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

[lhotari]

Thanks! Yes, this makes sense. Let's include this change in Pulsar 5.0.0 so that it becomes available. Pulsar 5.x client is backwards compatible with older broker versions.