Offset Set function should allow Offset as parameter.

[MatthL-SwiftC]

Description

When using the Offset.Set function I'd expect to be able to pass a value of the Offset type such as the predefined constant OffsetBeginning, however only a string or int64 representation is valid.

How to reproduce

Use Offset.Set with one of the predefined Offset constants.

Checklist

Please provide the following information:

• confluent-kafka-go and librdkafka version (LibraryVersion()): master
• Apache Kafka broker version: N/A
• Client configuration: N/A
• Operating system: N/A
• Provide client logs (with "debug": ".." as necessary) N/A
• Provide broker log excerpts N/A
• Critical issue