[feature request] Support SNAPPY CompressionType

## Motivation

As the title says. In go client, Snappy compression is not yet supported.

https://github.com/apache/pulsar-client-go/blob/77c7ccbd144b00d17c320c5f67cadaedb53f6b1e/pulsar/producer.go#L38-L43

Start with pulsar/producer.go at lines 38-43, then trace how the existing CompressionType values are represented and handled in the Go client. The work is done when SNAPPY is accepted and used consistently wherever producer compression types are processed; check the existing compression-related tests or test entry points before changing behavior.

Written by the indexing model from the issue text.