Add DeployTargetConfig and unmarshal config json

[Warashi]

What this PR does:

• Add struct DeployTarget to represent deploy targets for plugins.
• Implement unmarshaling of plugin config

Why we need it:

• I want the plugin developer to write less code, such as unmarshaling JSON to config struct.

Which issue(s) this PR fixes:

Part of #5530

Does this PR introduce a user-facing change?: No

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 38 lines in your changes missing coverage. Please review.

Project coverage is 26.29%. Comparing base (a281dcd) to head (14d1d7f).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/plugin/pipedsdk/deployment.go	0.00%	34 Missing ⚠️
pkg/plugin/pipedsdk/sdk.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5582      +/-   ##
==========================================
- Coverage   26.30%   26.29%   -0.01%     
==========================================
  Files         469      469              
  Lines       50323    50337      +14     
==========================================
  Hits        13235    13235              
- Misses      36027    36041      +14     
  Partials     1061     1061              

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[t-kikuc]

LGTM

(It took me a while to underdstand Config, DeployTarget, DeployTargetConfig 😅 )