[improve] refactor bulletin impl code

[tomsun28]

What's changed?

need to drop the hzb_bulletin table in db and restart

update readme and png

Checklist

• I have read the Contributing Guide
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

Add or update API

• I have added the necessary e2e tests and all cases have passed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 35.71429% with 27 lines in your changes missing coverage. Please review.

Project coverage is 32.00%. Comparing base (f5a8ebe) to head (8afca6c).

Files with missing lines	Patch %	Lines
...beat/manager/service/impl/BulletinServiceImpl.java	34.48%	12 Missing and 7 partials ⚠️
.../entity/manager/JsonMapListAttributeConverter.java	0.00%	4 Missing ⚠️
...rtzbeat/manager/controller/BulletinController.java	62.50%	3 Missing ⚠️
...ache/hertzbeat/common/entity/manager/Bulletin.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2714      +/-   ##
============================================
- Coverage     32.04%   32.00%   -0.05%     
+ Complexity     1693     1692       -1     
============================================
  Files           407      406       -1     
  Lines         16470    16438      -32     
  Branches       2130     2134       +4     
============================================
- Hits           5278     5261      -17     
+ Misses        10557    10539      -18     
- Partials        635      638       +3     

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