[fix][client] Make protobuf-java dependency optional in java client libraries

[Shawyeok]

Fixes apache#23469

Motivation

In the Pulsar Java client libraries, the protobuf-java library is only used in ProtobufSchema, making it suitable as an optional dependency. However, it is mistakenly included in the pulsar-client-admin and pulsar-client-all shaded jar and declared as required dependency of pulsar-client, pulsar-client-original and pulsar-client-admin-original, as noted by @merlimat in this discussion.

PR apache#23468 moved protobuf-java from a shaded JAR to a declared dependency in the pom.xml, which helped avoid conflicts between protobuf-java and application dependencies. Since users only need protobuf-java when using ProtobufSchema, it makes sense to declare it as an optional dependency. This change reduces the number of dependencies in the Pulsar client libraries.

Verifying this change

1. Execute the following Maven command:

mvn clean install -am -pl pulsar-client-all,pulsar-client-shaded,pulsar-client-admin-shaded -DskipTests

2. Verify that the protobuf-java dependency is not included in the installed pom.xml files.

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

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

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