What this PR does / why we need it:

Background

usualy when we develop a new feature on website or miniprogram or mobile application, we go through 3 steps:

1. design api documents,include api path,paramter,response;
2. develop frontend and backend at the same time;
3. testing and debugging the frontend and backend program.

in step 2, the frontend need to mock the api’s parameter and response,to debug program,and if there’s multiple front-end applications,each app need to do mock.

consider this scenario,if our api gateway can support api mocking,the steps 2 will come down to:

1. turn on mocking plugin when backend is still developing apis.
2. turn off mocking plugin when the api is finished.
   and there is nothing need to do for frontend developer,even neednt to change the api url.

based on the above scenario, the mocking plugin can be use in more scenario,such as:
when we pressure test the APISIX. we can use the mocking plugins to mock duration of api and status and response,instead of real upstream.

in microservice invocation chain,we can mock service one by one to find the  abnormal service.
Plugin Design

this plugin design is reference to mock.js and kong's mocking plugin.
it is planned to be implemented in two phases.

phase 1, goal to implement base mock abilities.
the general design is as follows:
local schema = {
type = "object",
properties = {
-- specify response delay time,default 0ms
delay = { type = "integer" },
-- specify response status,default 200
response_status = { type = "integer" },
-- specify response content type,support application/xml,text/plain and application/json,default application/json
content_type = { type = "content_type" },
-- specify response body.
response_example = {type = "string"},
-- specify response json schema,if response_example is not nil,this conf will be ignore.
-- generate random response by json schema.
response_schema = { type = "object" },
},
anyOf = {
{required = {"response_example"}},
{required = {"response_schema"}}
}
}

phase 2, goal to implement higher-order ability,such as "expectation and response patterns" and file download.

Related Discussion is #5896
The mail link is https://lists.apache.org/thread/xbkwz2k1dpoxgpcs7gm8t8d6ddvq1dto

Pre-submission checklist:

Did you explain what problem does this PR solve? Or what new features have been added?

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

others

What do you think about this idea？
i just commit the plugin code and chinese document now, If the idead works, i can attach else neccessary.