Refactor and improve fallback support for @SentinelResource annotation

[sczyh30]

Describe what this PR does / why we need it

Refactor and improve fallback support for @SentinelResource annotation to improve usability.

Does this pull request fix one issue?

Resolves #647

Describe how you did it

• Refactor logic for blockHandler and fallback
• Add fallbackClass support for fallback in global classes
• Add defaultFallback support

Breaking changes: the behavior of fallback has changed. In previous versions, the fallback will take effect only when blockHandler is not configured and DegradeException caught, which might be confusing and inconvenient for users. So I improved the semantics of fallback: now it can handle all kinds of exceptions (except configured exceptionsToIgnore). And a defaultFallback is also added to support universal fallback shared for multiple methods.

The method signature requirement of fallback has changed. Now it can be:

• return type should match the origin method;
• parameter list should match the origin method, and an additional Throwable parameter can be provided to get the actual exception.

For defaultFallback, the requirement:

• return type should match the origin method;
• parameter list should be empty, and an additional Throwable parameter can be provided to get the actual exception.

For BlockException, when blockHandler is provided, Sentinel will use the blockHandler, or Sentinel will try to use fallback to handle the BlockException.

Describe how to verify it

Run the test cases and demo.

Special notes for reviews

This PR contains breaking changes. Please take care of the changes.

[codecov-io]

Codecov Report

Merging #693 into master will increase coverage by 0.9%.
The diff coverage is 71.76%.

@@             Coverage Diff             @@
##             master     #693     +/-   ##
===========================================
+ Coverage     41.17%   42.08%   +0.9%     
- Complexity     1315     1398     +83     
===========================================
  Files           288      289      +1     
  Lines          8478     8771    +293     
  Branches       1147     1257    +110     
===========================================
+ Hits           3491     3691    +200     
- Misses         4551     4630     +79     
- Partials        436      450     +14

Impacted Files	Coverage Δ	Complexity Δ
...otation/aspectj/AbstractSentinelAspectSupport.java	69.17% <70.42%> (+5.17%)	38 <17> (+8)	⬆️
...l/annotation/aspectj/ResourceMetadataRegistry.java	79.16% <75%> (-4.17%)	12 <3> (+2)
...nel/annotation/aspectj/SentinelResourceAspect.java	83.33% <83.33%> (-0.88%)	5 <0> (+2)
...ntinel/slots/block/flow/param/ParameterMetric.java	68.25% <0%> (-6.75%)	37% <0%> (+8%)
...sentinel/slots/block/flow/param/ParamFlowRule.java	69.29% <0%> (-0.84%)	44% <0%> (+17%)
.../java/com/alibaba/csp/sentinel/util/SpiLoader.java	0% <0%> (ø)	0% <0%> (ø)	⬇️
...ots/statistic/ParamFlowStatisticEntryCallback.java	0% <0%> (ø)	0% <0%> (ø)	⬇️
.../command/handler/FetchTopParamsCommandHandler.java	0% <0%> (ø)	0% <0%> (?)
...a/csp/sentinel/slots/statistic/base/LeapArray.java	70.58% <0%> (+2.94%)	34% <0%> (+1%)	⬆️
...tinel/slots/block/flow/param/ParamFlowChecker.java	55.86% <0%> (+3.16%)	47% <0%> (+19%)	⬆️
... and 6 more

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[CarpenterLee]

LGTM