Add API gateway adapter common module and Spring Cloud Gateway adapter

[sczyh30]

Describe what this PR does / why we need it

Add an API gateway adapter common module, as well as Sentinel Spring Cloud Gateway adapter module and implementation.

Does this pull request fix one issue?

Resolves #86

Describe how you did it

• Add sentinel-api-gateway-adapter-common module for universal gateway rule and API definition management
• Add Sentinel Spring Cloud Gateway adapter module and implementation (based on sentinel-api-gateway-adapter-common and sentinel-reactor-adapter)

Describe how to verify it

Run the test cases and demo.

Special notes for reviews

NONE

[codecov-io]

Codecov Report

Merging #695 into master will decrease coverage by 0.1%.
The diff coverage is 39.41%.

@@             Coverage Diff              @@
##             master     #695      +/-   ##
============================================
- Coverage     41.38%   41.28%   -0.11%     
- Complexity     1236     1319      +83     
============================================
  Files           266      288      +22     
  Lines          7969     8478     +509     
  Branches       1091     1147      +56     
============================================
+ Hits           3298     3500     +202     
- Misses         4258     4545     +287     
- Partials        413      433      +20

Impacted Files	Coverage Δ	Complexity Δ
...pter/gateway/common/api/ApiPredicateGroupItem.java	0% <0%> (ø)	0 <0> (?)
...er/gateway/sc/callback/GatewayCallbackManager.java	0% <0%> (ø)	0 <0> (?)
...gateway/common/api/matcher/AbstractApiMatcher.java	0% <0%> (ø)	0 <0> (?)
...dapter/gateway/sc/route/RegexRoutePathMatcher.java	0% <0%> (ø)	0 <0> (?)
...xception/SentinelGatewayBlockExceptionHandler.java	0% <0%> (ø)	0 <0> (?)
...teway/sc/callback/RedirectBlockRequestHandler.java	0% <0%> (ø)	0 <0> (?)
...apter/gateway/common/param/GatewayParamParser.java	0% <0%> (ø)	0 <0> (?)
...ateway/sc/callback/DefaultBlockRequestHandler.java	0% <0%> (ø)	0 <0> (?)
...SpringCloudGatewayApiDefinitionChangeObserver.java	100% <100%> (ø)	2 <2> (?)
...pter/gateway/common/rule/GatewayRuleConverter.java	100% <100%> (ø)	2 <2> (?)
... and 37 more

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[CarpenterLee]

LGTM