Processor规则支持默认值

[saintping]

Is your feature request related to a problem? Please describe.
Processor设置规则的地方，和SQL语义很像。有select，from，where，into等语义，也支持简单的内置函数。为了方便业务逻辑，有需要支持默认值

Describe the solution you'd like
可以参考SQL的即as语义实现，比如 "select 2019 as year, filed from ...”