Add a Configuration for Default Schema

[reata]

Is your feature request related to a problem? Please describe.
When handling SQL like SELECT * FROM tab, sqllineage will assign <default> as the schema name. It would be user-friendly to be able to set that default value.

Describe the solution you'd like
Create a new environment variable as DEFAULT_SCHEMA.

Describe alternatives you've considered
Environment variable makes each process share the same default schema. It would be preferable to config through LineageRunner, but that requires a lot of change. So we'll start with environment variable.

[delphisharp]

Dear reata:
在实际生产中，不同的SQL语句有不同的模式名。
我不确认使用环境变量能否在多进程，多线程的方式下正常工作。
我已经完成修改并完成测试，稍后我会提交PR

In actual production, different SQL statements have different schema name.
I'm not sure whether using environment variables can work properly in multi-process and multi-thread mode.
I have completed the modification and completed the testing, I will submit a PR later

在设计中，各个类没有主基类，导致参数无法自由传递。
我考虑过全局变量，但也是担心在多进程，多线程的问题，故改为在结果汇集方法中进行批量修改。

In the design, each class does not have a main base class, so parameters cannot be passed freely.
I considered global variables, but I was also worried about the problem of multi-process and multi-threading, so I changed to batch modification in the result collection method.

[maoxingda]

May I ask if there is any multithreading code in the current code repository?

[delphisharp]

Current code repository means sqlineage's repository? If yes, then no.
But in my production environment, multiple processes are used to call sqllineage for data lineage

请问current code repository的意思是 sqlineage吗？如果是的话，那是没有的。
在我的生产环境，是使用多进程调用 sqllineage获取血缘的。多进程调用对大数据量处理很重要。

[maoxingda]

yes

[delphisharp]

在sqllineage没有多进程的测试代码
There is no multi-process test code in sqllineage

如果需要的话，我可以写个范例。
因为我提交了PR 修复这个问题，所以要不先看看 PR是否可以解决？
I can write an example if needed.
Since I submitted a PR to fix this problem, why not see if the PR can be solved first？

[reata]

Let me check the code first, thanks. If we can do it without being invasive, config through LineageRunner is without doubt the better approach.
我先看一下实现，代码侵入性不强的话，通过LineageRunner来配置肯定是更优的选择。

P.S: I like the Google Translate approach, thumb up! Currently we have more contributors who speak English only, so it's important we make sure every discussion is at least readable for them, thanks for your cooperation. If there's any details you'd like to talk, you can also reach me on email.
另，Google翻译的方式挺好的，赞！这个项目总体还是英文的contributor更多一些，所有的讨论留有英文还是很重要的，感谢你们的配合。细节上有不确定也欢迎邮件沟通。

[reata]

Close this issue and move configure through LineageRunner to #536