config center support for disable local cache

[wangdongyun]

关联的issue

1. sdk 支持禁用本地缓存 [config] sdk 支持禁用本地缓存 #563
2. sdk 在远程nacos不可用情况下，允许降级本地配置 [config] 在nacos故障场景下，client 无法初始化，降级本地配置失败 #564
3. 初始化 connection的时候，传AppName(同java-sdk 一致)，服务端方便统计

[codecov-commenter]

Codecov Report

Base: 31.26% // Head: 31.03% // Decreases project coverage by -0.24% ⚠️

Coverage data is based on head (5529615) compared to base (3b038b4).
Patch coverage: 0.00% of modified lines in pull request are covered.

❗ Current head 5529615 differs from pull request most recent head 77ed42b. Consider uploading reports for the commit 77ed42b to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #565      +/-   ##
==========================================
- Coverage   31.26%   31.03%   -0.24%     
==========================================
  Files          40       40              
  Lines        2987     3003      +16     
==========================================
- Hits          934      932       -2     
- Misses       1988     2004      +16     
- Partials       65       67       +2     

Impacted Files	Coverage Δ
clients/config_client/config_client.go	37.83% <0.00%> (-0.92%)	⬇️
clients/config_client/config_proxy.go	5.30% <0.00%> (-0.30%)	⬇️
common/constant/client_config_options.go	80.51% <0.00%> (-3.27%)	⬇️
common/nacos_server/nacos_server.go	16.98% <0.00%> (ø)
.../naming_client/naming_cache/service_info_holder.go	25.26% <0.00%> (-2.11%)	⬇️

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[binbin0325]

LGTM