Add appType property field in SentinelConfig

[sczyh30]

Describe what this PR does / why we need it

Add an appType property field in SentinelConfig which can be retrieved from csp.sentinel.app.type argument. It's useful to identify the service type (e.g. API gateway).

Does this pull request fix one issue?

NONE

Describe how you did it

Add an appType property field in SentinelConfig. It will be parsed during initialization of SentinelConfig. If the provided value is invalid or empty, it will be set to 0 (common) by default.

Describe how to verify it

Run the test cases.

Special notes for reviews

None.

[codecov-io]

Codecov Report

Merging #696 into master will increase coverage by 0.01%.
The diff coverage is 52.17%.

@@             Coverage Diff              @@
##             master     #696      +/-   ##
============================================
+ Coverage     41.17%   41.19%   +0.01%     
- Complexity     1315     1318       +3     
============================================
  Files           288      288              
  Lines          8478     8496      +18     
  Branches       1147     1149       +2     
============================================
+ Hits           3491     3500       +9     
- Misses         4551     4560       +9     
  Partials        436      436

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/com/alibaba/csp/sentinel/Constants.java	100% <ø> (+16.66%)	1 <0> (ø)	⬇️
...om/alibaba/csp/sentinel/config/SentinelConfig.java	51.61% <52.17%> (-3.8%)	14 <2> (+1)
.../slots/block/flow/controller/WarmUpController.java	82.69% <0%> (+3.84%)	11% <0%> (+1%)	⬆️

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[CarpenterLee]

LGTM