[feature request] Support KeyValue Schema.

### Motivation

CPP client supported `KeyValue` schema on https://github.com/apache/pulsar-client-cpp/pull/22.

Python clients also can support KeyValue schemas based on this.

### Alternatives

Note: The schema function of the Python client is not completely based on CPP. We may need to redesign the interface to the Key Value Schema in the Python client.

### Anything else?

先檢視 Python 用戶端現有的 schema 函式，以及 pull request 22 中參照的 C++ KeyValue schema 實作。確定 KeyValue schema 所需的 Python 介面，然後定義完成標準，使 Python 用戶端能夠一致地建立和使用這些 schema。

由索引模型根據 Issue 內容生成。