In the hotspot parameter rule, ParamKey is added to cooperate with the context attachments map to quickly specify the value that requires hotspot current limit from a large number of key-value pairs.
(#360)

Describe what this PR does / why we need it

It is convenient to specify hot spot parameters in hot spot current limiting rules. For example, there are a lot of data for system parameters, calibration parameters in header, and business parameters, and the parameters required by different businesses are different. It is very convenient to add all the data into ctx.attachments and specify the key in the rules.

Does this pull request fix one issue?

This time pr has added ParamKey, and modified the corresponding unit test and example.

Describe how you did it

ParamKey is added to the rule; trafficShaper also implements the extractArg interface correspondingly, which can be used with the existing paramIndex to easily specify hotspot parameters.

Describe how to verify it

Verified in unit test and example

Special notes for reviews

ParamKey can be considered as []string, so that users can specify hotspot rules flexibly in addition to specifying a single parameter.
For example, ParamKey=[]string{"uid","merchant_id"} This allows uid and merchant_id to be used as joint parameters as hot spots.