[reconstruction] change configFilterChain from public sigleton to private owned by ConfigClient

[robynron]

the IConfigFilterChain interface has only one default implementation global, and all NacosConfigClient instance objects depend on this global variable. That means the different NacosConfigClient will share the only one global variable, which will cause coupling effect to another NacosConfigClient when there are more than one NacosConfigClient.

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IConfigFilterChain接口只有一个默认的全局变量，而且不同的NacosConfigClient实例对象都依赖这个全局变量。这意味着不同的NacosConfigClient将共享唯一的一个全局变量，当有多个NacosConfigClient时，这将导致与另一个NacosConfigClient的耦合效应。

[codecov-commenter]

Codecov Report

Attention: 42 lines in your changes are missing coverage. Please review.

Comparison is base (8677a56) 29.81% compared to head (007a883) 31.10%.
Report is 1 commits behind head on master.

❗ Current head 007a883 differs from pull request most recent head 2ded645. Consider uploading reports for the commit 2ded645 to get more accurate results

Files	Patch %	Lines
clients/cache/disk_cache.go	56.92%	24 Missing and 4 partials ⚠️
clients/config_client/config_client.go	52.00%	10 Missing and 2 partials ⚠️
clients/config_client/config_proxy.go	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #700      +/-   ##
==========================================
+ Coverage   29.81%   31.10%   +1.29%     
==========================================
  Files          41       41              
  Lines        3153     3186      +33     
==========================================
+ Hits          940      991      +51     
+ Misses       2140     2116      -24     
- Partials       73       79       +6     

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