Provide Description for Sending Messages to a Topic that has a Schema Definition

**Is your feature request related to a problem? Please describe.**
I'd like to produce to a topic that has a defined schema with the Go client, but it's unclear whether this is possible, due to a lack of documentation.
I've noticed that a solution for schema _creation_ is being worked out, but is it even possible to send on a topic that has an existing schema currently?

**Describe the solution you'd like**
A clear description of how to send messages using a topic with a previously-defined (or client-defined) schema

**Describe alternatives you've considered**
I've noticed that there is a test file [here](https://github.com/apache/pulsar-client-go/blob/45c522d1e4ade7908a7c43b64ec567ee392259db/pulsar/schema_test.go) that seems to accomplish what I'd like to do, but it's pretty unclear, and I understand that it's not meant to be outwardly descriptive.. I'd just like to do the same if possible.

***Thank you!!***

Start with pulsar/schema_test.go, which the issue identifies as an example of sending messages with a schema. Use it to determine whether the Go client supports topics with existing or client-defined schemas, then document the required steps and usage clearly. Done means a newcomer can follow the documentation to produce messages to a schema-defined topic.

Written by the indexing model from the issue text.