feat: export ServerHealthy in INamingClient

[lixiangwuxian]

为 INamingClient 接口提供 ServerHealthy() bool 方法，便于无副作用的进行nacos服务可用性检查 fix:#676

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[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (4eb0559) 29.71% compared to head (b60e9ae) 29.82%.

❗ Current head b60e9ae differs from pull request most recent head cd60601. Consider uploading reports for the commit cd60601 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #677      +/-   ##
==========================================
+ Coverage   29.71%   29.82%   +0.11%     
==========================================
  Files          40       40              
  Lines        3093     3095       +2     
==========================================
+ Hits          919      923       +4     
+ Misses       2104     2103       -1     
+ Partials       70       69       -1     

Files	Coverage Δ
clients/naming_client/naming_client.go	34.32% <0.00%> (-0.35%)	⬇️

... and 2 files with indirect coverage changes

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[binbin0325]

LGTM