feat: add message api

[eddycharly]

Explanation

Add message api.

[codecov]

Codecov Report

Attention: Patch coverage is 44.77612% with 37 lines in your changes missing coverage. Please review.

Project coverage is 23.40%. Comparing base (a724195) to head (ac831f5).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/apis/policy/v1alpha1/message.go	0.00%	19 Missing ⚠️
pkg/message/message.go	70.00%	7 Missing and 5 partials ⚠️
pkg/apis/policy/v1alpha1/zz_generated.deepcopy.go	0.00%	4 Missing ⚠️
pkg/matching/match.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #487      +/-   ##
==========================================
+ Coverage   23.18%   23.40%   +0.21%     
==========================================
  Files          86       88       +2     
  Lines        4058     4106      +48     
==========================================
+ Hits          941      961      +20     
- Misses       3032     3059      +27     
- Partials       85       86       +1     

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