[type:fix] Add SPI @Join annotation to the isSingleton() property

[prFor]

Make sure that:

• [V] You have read the contribution guidelines.
• [V] You submit test cases (unit or integration tests) that back your changes.
• [V] Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

[codecov-commenter]

Codecov Report

Merging #3955 (4792d71) into master (14ec556) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #3955      +/-   ##
============================================
- Coverage     70.16%   70.14%   -0.03%     
+ Complexity     6678     6677       -1     
============================================
  Files           923      923              
  Lines         25071    25067       -4     
  Branches       2264     2264              
============================================
- Hits          17590    17582       -8     
- Misses         6112     6120       +8     
+ Partials       1369     1365       -4     

Impacted Files	Coverage Δ
...in/java/org/apache/shenyu/spi/ExtensionLoader.java	86.58% <100.00%> (-0.92%)	⬇️
...controller/ShenyuClientHttpRegistryController.java	77.77% <0.00%> (-22.23%)	⬇️
...ruptor/RegisterClientServerDisruptorPublisher.java	52.94% <0.00%> (-11.77%)	⬇️
...shenyu/sync/data/consul/ConsulSyncDataService.java	80.00% <0.00%> (+1.66%)	⬆️

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