The roadmap of the golang client

Since the initial implementation of the golang client has been merged(#19), there are still some unfinished tasks to be done.

In the first stage, the basic features will be implemented.

* **Adapt to [v2 protocol](https://github.com/apache/rocketmq-apis)**
The [v1 version](https://github.com/apache/rocketmq-apis/releases/tag/v1.0) is archived. It is necessary to adapt to the latest protocol.
* **Adapt to the unified API specs ( Producer + Simple Consumer )**
We established a [New and Unified API Specs](https://shimo.im/docs/m5kv92OeRRU8olqX), see more details from [java apis](https://github.com/apache/rocketmq-clients/tree/master/java/client-apis/src/main/java/org/apache/rocketmq/client/apis). The part of producer and simple consumer takes precedence. The four types of message will be supported in this phase.

In the second stage.
* **Clear error handling**
The [RPC-status code mapping table](https://github.com/apache/rocketmq-apis/blob/main/status_code.md) indicates the status codes that may occur in various cases across languages, you can get more information from this.
* **Improve the observability**
The observability contains logging, tracing and metrics, you can improve observability of client-side through [OpenTelemetry](https://opentelemetry.io/) or other tools from these aspects.
* **Adapt to the unified API specs ( Push Consumer )**
Implement the push consumer.

Documentations and unit tests should cover the corresponding part in each phase.

Start with the initial Go client implementation from issue #19, then compare it with the v2 protocol and the Java client-apis referenced here. Break the roadmap into separately scoped protocol, unified API, error-handling, observability, and push-consumer work. Done requires the corresponding implementation, documentation, and unit-test coverage for each phase.

Written by the indexing model from the issue text.