support to register external object fecthers in metaserver

[waynepeking348]

What type of PR is this?

enhancements

What this PR does / why we need it:

support to register external object fecthers in metaserver

[codecov]

Codecov Report

Patch coverage: 85.71% and project coverage change: +0.11 🎉

Comparison is base (2cc0d49) 50.14% compared to head (77d5069) 50.26%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #163      +/-   ##
==========================================
+ Coverage   50.14%   50.26%   +0.11%     
==========================================
  Files         408      408              
  Lines       38760    38772      +12     
==========================================
+ Hits        19438    19489      +51     
+ Misses      17139    17098      -41     
- Partials     2183     2185       +2     

Flag	Coverage Δ
unittest	50.26% <85.71%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/metaserver/agent/agent.go	75.00% <85.71%> (-1.39%)	⬇️

... and 10 files with indirect coverage changes

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