Key-based batching and hashingScheme parameter for Python client

**Short intro**
We're running a platform that's effectively just a bunch of microservices written in Python, Java and Go which are communicating through Kafka. Currently we're considering moving from Kafka to Pulsar.
However, one of the main features we need is key-based message routing for solving concurrency problems. Pulsar has that option with KeyShared subscription mode.

**The problem**
In Python, KeyShared subscription works only if producer disables batching. I ran some tests in my local environment and disabling batching results in high decrease of throughput (~25k/s with batching vs ~9k/s without).

**Features that would've solved the problem**
- Key-based batching for Python client
- HashingScheme parameter when creating producer so all of our services (Java, Python and Go) could have the same hashing scheme

首先追踪 Python 客户端目前如何处理 producer batching，以及 producer 创建如何暴露配置。比较 Python、Java 和 Go 客户端使用的哈希方案。完成的标准是：Python 支持基于 key 的 batching，并且 producer 可以选择一种能够在这些客户端之间生成兼容路由的哈希方案。

由索引模型根据 Issue 内容生成。