add fallback router.

[provenceee]

【Fix issue】 #1405

[Modification] Added fallback routing, this function has been implemented in 1.0

[Use case description] Added ut

[Self-test situation] ut passed, local test passed

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (9d1f090) 42.53% compared to head (305fce8) 42.82%.
Report is 12 commits behind head on develop.

Files	Patch %	Lines
...t/router/config/strategy/AbstractRuleStrategy.java	78.57%	1 Missing and 2 partials ⚠️
...eicloud/sermant/router/config/utils/RuleUtils.java	0.00%	3 Missing ⚠️
...sermant/router/dubbo/handler/FlowRouteHandler.java	83.33%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #1419      +/-   ##
=============================================
+ Coverage      42.53%   42.82%   +0.29%     
- Complexity      3307     3367      +60     
=============================================
  Files            831      845      +14     
  Lines          18528    18757     +229     
  Branches        2043     2075      +32     
=============================================
+ Hits            7880     8033     +153     
- Misses          9661     9727      +66     
- Partials         987      997      +10     

Flag	Coverage Δ
unittests	42.82% <78.78%> (+0.29%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.