[type:feat] support alert notice

[tomsun28]

support alert notice design:

• shenyu admin provider a alarm report api /alert/report to receiver alarm content from gateway pulgin
• gateway send alert message when them alert trigger
• shenyu dashboard support manage alert receiver name , alert type(email, dingding, wechat ...)

TODO:

• shenyu admin backend unit test and more
• a shenyu gateway plugin trigger and send alert to admin
• relate shenyu dashboard component ui
• doc and test

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

[codecov-commenter]

Codecov Report

Merging #4782 (f5382df) into master (84a6cf3) will decrease coverage by 0.58%.
The diff coverage is 9.04%.

@@             Coverage Diff              @@
##             master    #4782      +/-   ##
============================================
- Coverage     61.71%   61.13%   -0.58%     
+ Complexity     8495     8471      -24     
============================================
  Files          1232     1242      +10     
  Lines         36999    37317     +318     
  Branches       3514     3534      +20     
============================================
- Hits          22833    22815      -18     
- Misses        12203    12538     +335     
- Partials       1963     1964       +1     

Files Changed	Coverage Δ
...che/shenyu/admin/model/entity/AlertReceiverDO.java	0.00% <0.00%> (ø)
...e/shenyu/admin/model/query/AlertReceiverQuery.java	0.00% <0.00%> (ø)
...ava/org/apache/shenyu/common/dto/AlarmContent.java	0.00% <0.00%> (ø)
...g/apache/shenyu/plugin/base/alert/AlarmSender.java	0.00% <0.00%> (ø)
...che/shenyu/plugin/base/alert/AlarmServiceImpl.java	0.00% <0.00%> (ø)
...nyu/plugin/base/alert/AlarmThreadPoolExecutor.java	0.00% <0.00%> (ø)
...e/shenyu/web/configuration/RestTemplateConfig.java	0.00% <0.00%> (ø)
...u/admin/service/impl/AlertReceiverServiceImpl.java	2.43% <2.43%> (ø)
...enyu/admin/controller/AlertReceiverController.java	5.88% <5.88%> (ø)
...min/mybatis/handler/AbstractObjectTypeHandler.java	11.11% <11.11%> (ø)
... and 6 more

... and 4 files with indirect coverage changes

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[tomsun28]

relate shenyu dashboard pr apache/shenyu-dashboard#333