Improve the KafkaHub implementation

[ayeshLK]

Description

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Following tasks should be completed with this:

• Incorporate support to convert Kafka headers to HTTP headers
• Incorporate support to configure maximum messages for consumer polling (This can be configured using maxPollRecords configuration in Ballerina Kafka consumer)
• Incorporate support for parallel message delivery
• Partition based message consumption for subscribers
• Retrying for message delivery errors

[ayeshLK]

Completed the task with following PRs

• Incorporate support to convert Kafka headers to HTTP headers module-ballerina-websubhub#1051
• Incorporate support to configure maximum messages for consumer polling module-ballerina-websubhub#1052
• Add support for retrying for message delivery errors module-ballerina-websubhub#1053
• Add support for parallel message-delivery module-ballerina-websubhub#1059
• Add support for partition based message consumption for subscribers module-ballerina-websubhub#1060