add scheduler config and support out-of-tree scheduler plugin

[silenceper]

What type of PR is this?

• support scheduler configurable
• support add custom plugin

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #281

Special notes for your reviewer:

[codecov-commenter]

Codecov Report

Merging #498 (3641ea3) into main (76b0dd0) will increase coverage by 0.03%.
The diff coverage is n/a.

❗ Current head 3641ea3 differs from pull request most recent head 21e40d4. Consider uploading reports for the commit 21e40d4 to get more accurate results

@@            Coverage Diff             @@
##             main     #498      +/-   ##
==========================================
+ Coverage   13.07%   13.10%   +0.03%     
==========================================
  Files          65       65              
  Lines        7081     7079       -2     
==========================================
+ Hits          926      928       +2     
+ Misses       6077     6073       -4     
  Partials       78       78              

Impacted Files	Coverage Δ
...m/clusternet/clusternet/pkg/scheduler/scheduler.go	4.04% <0.00%> (-0.31%)	⬇️
...ub.com/clusternet/clusternet/pkg/utils/deployer.go	6.28% <0.00%> (-0.05%)	⬇️
...ithub.com/clusternet/clusternet/pkg/agent/agent.go	1.68% <0.00%> (-0.03%)	⬇️
...ithub.com/clusternet/clusternet/pkg/utils/names.go	100.00% <0.00%> (ø)
...ternet/clusternet/pkg/scheduler/default_plugins.go
...hub.com/clusternet/clusternet/pkg/utils/version.go	100.00% <0.00%> (ø)

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[silenceper]

/PTAL @dixudx

[dixudx]

Nice implementation. Good work!