[pulsar-client-cpp] Partition key from producer does arrive in broker

**Describe the bug**
When using the Java client producer, the setting of Key in MessageBuilder correctly coveys this metadata to the broker. C++ client consumers can then successfully use _Key_Shared_ subscription on this topic and we're seeing the expected keys-based consumption behavior.

However, when we use a C++ client producer, we do not get a "shared key" consumption behavior over the subscribed consumers.

**To Reproduce**
Steps to reproduce the behavior:
1. Initiate a Java client producer. Add a Key to the MessageBuidler.
2. Send messages using the MessageBuilder
3. Initiate two C++ consumers using the "Key_Shared" subscription
4. See key-based distributed messages over the consumers.
5. Now stop the Java client producer
6. Start a CPP client producer that sets the same metadata
7. Now only one of the C++ client consumers retrieves all messages

**Expected behavior**
We'd expect to see the same "key-shared" message distribution over the consumers with a C++ client producer as with its Java equivalent.

**Additional context**
We suspect a difference in the protobuf schema between the Java and C++ client producers for setting the partition or ordering key and conveying this metadata to the broker. It seems that the broker does not receive the message metadata from the C++ client producer correctly.

NB: We are using the same version of C++ client and broker (both from Pulsar v2.5.0).

Start with the C++ producer's MessageBuilder and compare how its key or ordering-key metadata is serialized with the Java producer behavior described in the reproduction steps. Reproduce the issue with two C++ Key_Shared consumers and both producer types using Pulsar 2.5.0. Done means the C++ producer produces the same key-based distribution across consumers as the Java producer.

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