MEP 18 -- Support Milvus 2.0 C++ SDK

Current state: Accepted

ISSUE: https://github.com/milvus-io/milvus/issues/7713

PRs: 

Keywords: C++ SDK

Released: with Milvus 2.1 

Authors:  

Summary(required)

Deliver C++ SDK with full functionality for Milvus 2.0.

Motivation(required)

We've seen many users demands for C++ SDK, it is probably the most useful SDK which could be used in distributed systems.

Public Interfaces(optional)

Design Details(required)

TODO lists

1. API Design → Reference Java 1.1 SDK, Pymilvus SDK
2. Initial version of the code, finish all the frameworks, including how the sdk communicate with milvus cluster, how we do unit test.
3. Finish DDL, DML , Control  API implementation
4. Unit Test 
5. Usage examples
6. API Document
7. CI Test

Which C++ versions are we going to support?

Any C++ version above C++10. The Reason is that :

1. Wider user range, as may organizations and devices support C++10.
2. Easy to maintain as it will get a large group of developer support.

Test Plan(required)

1. Unit test
   1. C++ SDK will implement a mock milvus for basic testing, need to be tested with mainstream distributions(e.g. Ubuntu 18.04+, CentOS 7+)
   2. Start a standalone milvus complicated test.
2. CI test 
   1. Do we need to setup basic CI test for further improvement?
3. Examples
   
   1. finish all the examples in user guide and make sure it works like https://milvus.io/docs/v2.0.0/example_code.md

Rejected Alternatives(optional)

References(optional)