Review incubator-shenyu #3599 pr

[刘天佑]

Dear community:

The  shenyu  has new pr , please review the code.

Here is this pr main function:

  1. The mock plugin support auto generate random data by config  ${} 
placeholder 

  2. add  unit test

Here is link :  https://github.com/apache/incubator-shenyu/pull/3599