Include support for API key "METADATA"

[aaron-613]

Bundled with the vanilla version of Apache Kafka is a very basic command line publisher tool: kafka-console-producer. This tool just attempts to publish to a topic whatever the user types in. However, the producer cannot connect to the proxy:

Producer console:

bin/kafka-console-producer.sh --bootstrap-server localhost:9094 --topic blah
>[2022-12-12 21:55:54,157] WARN [Producer clientId=console-producer] Bootstrap broker localhost:9094 (id: -1 rack: null) disconnected (org.apache.kafka.clients.NetworkClient)
[2022-12-12 21:55:54,260] WARN [Producer clientId=console-producer] Bootstrap broker localhost:9094 (id: -1 rack: null) disconnected (org.apache.kafka.clients.NetworkClient)
[2022-12-12 21:55:54,374] WARN [Producer clientId=console-producer] Bootstrap broker localhost:9094 (id: -1 rack: null) disconnected (org.apache.kafka.clients.NetworkClient)
...

Proxy console:

bin/kafkaproxy ../../../proxy-example.properties
[main] INFO org.apache.kafka.solace.kafkaproxy.ProxyReactor - Listening for incoming connections on PLAINTEXT localhost/127.0.0.1:9094
[Kafka_Proxy_Reactor] INFO org.apache.kafka.solace.kafkaproxy.ProxyReactor - New incoming connection request on PLAINTEXT localhost/127.0.0.1:9094 from /127.0.0.1:52312
[Kafka_Proxy_Reactor] INFO org.apache.kafka.solace.kafkaproxy.ProxyChannel - Cleaning up channel (remote /127.0.0.1:52312) due to Request parse did not work: org.apache.kafka.common.errors.InvalidRequestException: Invalid API key METADATA, expecting handshake request
[Kafka_Proxy_Reactor] INFO org.apache.kafka.solace.kafkaproxy.ProxyReactor - New incoming connection request on PLAINTEXT localhost/127.0.0.1:9094 from /127.0.0.1:52314
[Kafka_Proxy_Reactor] INFO org.apache.kafka.solace.kafkaproxy.ProxyChannel - Cleaning up channel (remote /127.0.0.1:52314) due to Request parse did not work: org.apache.kafka.common.errors.InvalidRequestException: Invalid API key METADATA, expecting handshake request
[Kafka_Proxy_Reactor] INFO org.apache.kafka.solace.kafkaproxy.ProxyReactor - New incoming connection request on PLAINTEXT localhost/127.0.0.1:9094 from /127.0.0.1:52316
[Kafka_Proxy_Reactor] INFO org.apache.kafka.solace.kafkaproxy.ProxyChannel - Cleaning up channel (remote /127.0.0.1:52316) due to Request parse did not work: org.apache.kafka.common.errors.InvalidRequestException: Invalid API key METADATA, expecting handshake request