improve the code of URLStrParser.java

[zhongxiongzeng]

What is the purpose of the change

1. URLStrParser#parseDecodedStr 和 URL#valueOf都是将String解析成URL，但是parseDecodedStr解析parameters时支持以；作为分割，valueOf不支持。此pr修改了valueOf方法，支持以；作为分隔符。
2. 优化URLStrParser#addParam方法。

Brief changelog

Verifying this change

Checklist

• Make sure there is a GitHub_issue field for the change (usually before you start working on it). Trivial changes like typos do not require a GitHub issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
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• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
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[codecov-commenter]

Codecov Report

Merging #8085 (d548761) into master (943d27b) will decrease coverage by 0.82%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master    #8085      +/-   ##
============================================
- Coverage     61.18%   60.36%   -0.83%     
+ Complexity      491      487       -4     
============================================
  Files          1091     1091              
  Lines         43953    43943      -10     
  Branches       6417     6416       -1     
============================================
- Hits          26892    26524     -368     
- Misses        14076    14438     +362     
+ Partials       2985     2981       -4     

Impacted Files	Coverage Δ
...ain/java/org/apache/dubbo/common/URLStrParser.java	85.02% <42.85%> (+1.41%)	⬆️
...mon/src/main/java/org/apache/dubbo/common/URL.java	55.02% <100.00%> (ø)
...ookeeper/ZookeeperDynamicConfigurationFactory.java	0.00% <0.00%> (-100.00%)	⬇️
...gcenter/wrapper/CompositeDynamicConfiguration.java	0.00% <0.00%> (-73.34%)	⬇️
...fig/configcenter/TreePathDynamicConfiguration.java	25.71% <0.00%> (-60.01%)	⬇️
...pport/zookeeper/ZookeeperDynamicConfiguration.java	0.00% <0.00%> (-57.15%)	⬇️
.../dubbo/metadata/report/MetadataReportInstance.java	12.50% <0.00%> (-40.63%)	⬇️
.../configcenter/support/zookeeper/CacheListener.java	0.00% <0.00%> (-28.58%)	⬇️
...ng/transport/dispatcher/all/AllChannelHandler.java	62.06% <0.00%> (-27.59%)	⬇️
...g/apache/dubbo/remoting/p2p/support/FileGroup.java	41.50% <0.00%> (-22.65%)	⬇️
... and 53 more

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