peek message for pulsar-admin java client does not return a key of a peeked message

**Describe the bug**
For messages got via pulsarAdmin.topics().peekMessages(topic, subscription, total) method from pulsar-client-admin, getKey() method always returns an empty string.

**To Reproduce**
Steps to reproduce the behavior:
1. Write some messages with specified message keys to a topic
2. Call pulsarAdmin.topics().peekMessages(topic, subscription, total); (pulsarAdmin is an instance of org.apache.pulsar.client.admin.PulsarAdmin)
3. For a message from a list of message received on previous step call getKey() method
4. empty string will be returned

**Expected behavior**
getKey() method should return a message key.

**Screenshots**
E.g. of getKey() result for a peeked message:

E.g. of getKey() result for the same message which was received (with pulsar-client v2.7.2):

**Desktop (please complete the following information):**
we are using pulsar-client-admin v2.7.2
```

org.apache.pulsar
pulsar-client-admin
2.7.2

```

**Additional context**

A bit of a shot in the dark, but I think this happens because for peeked message getKey() returns a partiton key from metadata, but not the actual key itself.

See org.apache.pulsar.client.impl.MessageImpl:

AFAIK pulsar-admin java client uses pulsar-admin rest api, and API doesn't actually return a key, only metadata.getPartitionKey().
See https://github.com/apache/pulsar/blob/a2b7cae3cfe8d3776483b1ecaf69af47949b70e1/pulsar-broker/src/main/java/org/apache/pulsar/broker/admin/impl/PersistentTopicsBase.java#L2595

Though I can be completely wrong here

Start with pulsar-broker/src/main/java/org/apache/pulsar/broker/admin/impl/PersistentTopicsBase.java around line 2595 and inspect how peekMessages data is exposed through the REST API. Compare that response with org.apache.pulsar.client.impl.MessageImpl and the pulsarAdmin.topics().peekMessages entry point. Done means a peeked message's getKey() returns the original message key.

Written by the indexing model from the issue text.