Add data-source extension for OpenSergo fault-tolerance spec

[sczyh30]

Signed-off-by: Eric Zhao sczyh16@gmail.com

Describe what this PR does / why we need it

Add data-source extension for OpenSergo fault-tolerance specification, so that users could leverage unified OpenSergo CRDs to manage Sentinel rules.

Does this pull request fix one issue?

Resolves #2841

Describe how you did it

Basic design:

• OpenSergoRuleAggregator: gather all OpenSergo fault-tolerance rules and convert them to Sentinel rules

Current supported strategies:

• RateLimitStrategy (FlowRule, controlBehavior=0)
• ThrottlingStrategy (FlowRule, controlBehavior=2)
• ConcurrencyLimitStrategy (FlowRule, limitMode=concurrency)
• CircuitBreakerStrategy (DegradeRule)
• AdaptiveOverloadProtectionStrategy (SystemRule)

Describe how to verify it

A demo for creating and registering OpenSergo data-source:

OpenSergoDataSourceGroup openSergo = new OpenSergoDataSourceGroup(host, port, namespace, appName);
openSergo.start();

// Subscribe flow rules from OpenSergo control plane, and propagate to Sentinel rule manager.
FlowRuleManager.register2Property(openSergo.subscribeFlowRules());

Unit test TBD...

Special notes for reviews

This PR uses a beta version of opensergo-java-sdk. See https://github.com/opensergo/opensergo-java-sdk for details.

[kaori-seasons]

Is it possible to set a delayed start here, such as the following code segment:

ScheduledExecutorService singlonThread = Executors
.newSingleThreadScheduledExecutor();
while (!singlonThread.awaitTermination(3000,
TimeUnit.MILLISECONDS)) {
singleThread.shutdown();
}

This is done so that the user can manually set the timeout

[jnan806]

.