feat(trafiic-split): support rules based on json request body

[wilsonGen]

Description

After this change, the traffic-split will be able to support rules based on json body in POST request. For example:
If we create a route like this:

{
   "methods": [
      "POST"
   ],
   "name": "Route based on json body (POST)",
   "uri": "/*",
   "plugins": {
      "traffic-split": {
         "rules": [
            {
               "match": [
                  {
                     "vars": [
                        [
                           "json_body_arg_$.cat.sounds_like",
                           "==",
                           "meow"
                        ]
                     ]
                  }
               ],
               "weighted_upstreams": [
                  {
                     "upstream": {
                        "nodes": {
                           "httpbin.org:80": 1
                        },
                        "type": "roundrobin"
                     },
                     "weight": 3
                  }
               ]
            }
         ]
      }
   },
   "upstream": {
      "nodes": {
         "google.com": 1
      },
      "type": "roundrobin"
   }
}

In traffic-split we defined "json_body_arg_cat.sounds_like" == "mewo", which means it will match the json body like this:

{
    "cat": {
        "sounds_like": "meow"
    },
    "other_fields": "hihi"
}

Motivation:
Currently the traffic-split plugin can create rules based on the default form of the POST request.
However it does not support rules based on application/json in the POST request body.
I think it will be great if it can support the rules based on json body as well.
As I have the need to have rules based on json body too.
Fixes # (issue)

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[shreemaan-abhishek]

please add test cases and docs

[wilsonGen]

I have added test cases and docs, please let me know if there are anything missing, thank you. 😃

[bzp2010]

Suggested change

	local body = cjson.decode(raw_request_body)
	local body, err = core.json.decode(raw_request_body)

[bzp2010]

Can we use jsonpath?

[wilsonGen]

yes, just updated code using jsonpath, thank you. 😃

[soulbird]

Consider using a cache, as this code may be executed multiple times on hot paths.

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