Improve the SPI ClassLoader mechanism to handle more complex scenarios

[sczyh30]

Describe what this PR does / why we need it

Improve the SPI ClassLoader mechanism to handle more complex scenarios.

Does this pull request fix one issue?

Resolves #1087

Describe how you did it

• Add a property csp.sentinel.spi.classloader to indicate which ClassLoader should be used when loading SPI implementations. By default Sentinel will use the default ClassLoader (i.e. clazz.getClassLoader()). Only if the property is set to context, Sentinel will try to load SPI implementations with the thread context ClassLoader.
• Add a ServiceLoaderUtil class as the universal ServiceLoader wrapper, and improve the SpiLoader.
• Change all SPI providers to use ServiceLoaderUtil instead.

Describe how to verify it

Run all test cases and demo.

Special notes for reviews

This PR contains internal breaking change. Please verify it in scenarios with advanced ClassLoader mechanism.

[codecov-io]

Codecov Report

Merging #1088 into master will increase coverage by 0.06%.
The diff coverage is 47.22%.

@@             Coverage Diff              @@
##             master    #1088      +/-   ##
============================================
+ Coverage      42.7%   42.76%   +0.06%     
- Complexity     1491     1494       +3     
============================================
  Files           319      320       +1     
  Lines          9378     9383       +5     
  Branches       1279     1278       -1     
============================================
+ Hits           4005     4013       +8     
+ Misses         4878     4875       -3     
  Partials        495      495

Impacted Files	Coverage Δ	Complexity Δ
...om/alibaba/csp/sentinel/config/SentinelConfig.java	57.74% <ø> (ø)	14 <0> (ø)	⬇️
.../sentinel/cluster/server/TokenServiceProvider.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...ter/server/processor/RequestProcessorProvider.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...va/com/alibaba/csp/sentinel/init/InitExecutor.java	60.52% <100%> (ø)	6 <0> (ø)	⬇️
...baba/csp/sentinel/slotchain/SlotChainProvider.java	70% <100%> (+6.84%)	3 <0> (-2)	⬇️
.../java/com/alibaba/csp/sentinel/util/SpiLoader.java	16.32% <47.61%> (+7.68%)	4 <2> (+2)	⬆️
...om/alibaba/csp/sentinel/spi/ServiceLoaderUtil.java	60% <60%> (ø)	2 <2> (?)
...a/csp/sentinel/slots/statistic/base/LeapArray.java	67.32% <0%> (-2.98%)	33% <0%> (-1%)
...p/sentinel/datasource/consul/ConsulDataSource.java	69.56% <0%> (+4.34%)	7% <0%> (ø)	⬇️
... and 1 more

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[ghost]

LGTM