Add Sentinel Envoy RLS server implementation

[sczyh30]

Describe what this PR does / why we need it

Add the sentinel-cluster-server-envoy-rls module, a Envoy RLS server implementation using Sentinel token server. See #666 (comment) for initial design proposal.

Does this pull request fix one issue?

Resolves #666

Describe how you did it

• EnvoyRlsRuleManager: loads EnvoyRlsRule, convert to Sentinel cluster flow rules and apply to ClusterFlowRuleManager (in default namespace).
• EnvoySentinelRuleConverter: convert EnvoyRlsRule to cluster FlowRule and generate a consistent and unique flow ID.
• SentinelEnvoyRlsServiceImpl: the RLS gRPC service implementation, which will delegate rule checking to SimpleClusterFlowChecker and assemble result for all descriptors.
• EnvoyRlsRuleDataSourceService: for initializing the YAML file rule data source.
• SentinelEnvoyRlsServer: the main entry of the RLS server.

Describe how to verify it

Run the test cases and demo.

Special notes for reviews

NONE

[codecov-io]

Codecov Report

Merging #1139 into master will decrease coverage by 0.17%.
The diff coverage is 36.42%.

@@             Coverage Diff              @@
##             master    #1139      +/-   ##
============================================
- Coverage     42.93%   42.75%   -0.18%     
- Complexity     1503     1528      +25     
============================================
  Files           320      329       +9     
  Lines          9403     9683     +280     
  Branches       1283     1314      +31     
============================================
+ Hits           4037     4140     +103     
- Misses         4871     5035     +164     
- Partials        495      508      +13

Impacted Files	Coverage Δ	Complexity Δ
...erver/envoy/rls/flow/SimpleClusterFlowChecker.java	0% <0%> (ø)	0 <0> (?)
.../rls/datasource/EnvoyRlsRuleDataSourceService.java	0% <0%> (ø)	0 <0> (?)
...uster/server/envoy/rls/SentinelEnvoyRlsServer.java	0% <0%> (ø)	0 <0> (?)
...luster/server/envoy/rls/SentinelRlsGrpcServer.java	0% <0%> (ø)	0 <0> (?)
...ter/server/envoy/rls/rule/EnvoyRlsRuleManager.java	41.07% <41.07%> (ø)	4 <4> (?)
.../cluster/server/envoy/rls/log/RlsAccessLogger.java	45.45% <45.45%> (ø)	2 <2> (?)
.../server/envoy/rls/SentinelEnvoyRlsServiceImpl.java	51.78% <51.78%> (ø)	7 <7> (?)
...el/cluster/server/envoy/rls/rule/EnvoyRlsRule.java	57.5% <57.5%> (ø)	5 <5> (?)
...ver/envoy/rls/rule/EnvoySentinelRuleConverter.java	84.61% <84.61%> (ø)	6 <6> (?)
... and 9 more

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[ghost]

lgtm