[EPIC] Design and implement generic connector interface

[jerryshao]

The metadata connector system is the core system in Graviton to describe the connector how to create, and run, including SPI(service provider interface), different data source's field type mapping, and memory structure.

This umbrella issue tracks the whole design and implementation of Graviton's metadata connector(not data connector) system, including:

1. Connector factory design & implementation.
2. Connector extract metadata job schedule design & implementation.
3. Connector plug-in framework.
4. Unified metadata type definite.
5. Type converter design & implementation.
6. Metadata multiple version design & implementation.
7. Typical connector design & implementation.

Subtasks are list in here:

• [Subtask] Metadata Connector Spec design #19
• [#24] Add metadata connector plugin framework #28

[jerryshao]

I assign this issue to you @xunliu

[jerryshao]

You'd better list all the subtasks in the issue description like I did in #3 .

[jerryshao]

This is not necessary any more.