feat(alerting): custom alert support endpoint errors

[raojinlin]

Summary

Fixes #840

• Updated buildHTTPRequest method in AlertProvider to accept a result parameter.
• Added support for including [ENDPOINT_ERRORS] in both the request body and URL, which will be replaced by the errors from Result.Errors[].
• Modified the Send method to pass the result object to buildHTTPRequest.
• Adjusted CreateExternalEndpointResult to capture and store errors from query parameters.
• This allows custom alerts to include detailed error information, enhancing the flexibility of alert notifications.

Usage

push the status of an external endpoint

curl -X POST -H "Authorization: Bearer 123456" "http://127.0.0.1:8080/api/v1/endpoints/core_ext-ep-test/external?success=false&error=xxxx"

custom alerts config

alerting:
  custom:
    url: https://qyapi.weixin.qq.com/cgi-bin/webhook/send?key=xxxx  
    method: POST
    headers:
      Content-Type: application/json
    body: |
      {
        "msgtype": "markdown",
        "markdown": {
          "content": "Gatus:\n <font color='warning'>[ALERT_TRIGGERED_OR_RESOLVED]</font>: [ENDPOINT_GROUP] - [ENDPOINT_NAME] - <font color='commen'>[ALERT_DESCRIPTION]</font>\n<font color='warning'>[ENDPOINT_ERRORS]</font>"
        }
      }

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Updated documentation in README.md, if applicable.

[raojinlin]

Hi @TwiN I've made the updates as per your suggestions. Could you please take another look? Thanks!

[gh0st3e]

hey, appreciate you work, i also really need this feature, how about to merge it

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.98%. Comparing base (b0c2f1e) to head (2d5aeb8).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #844      +/-   ##
==========================================
+ Coverage   78.96%   78.98%   +0.01%     
==========================================
  Files          71       71              
  Lines        4768     4772       +4     
==========================================
+ Hits         3765     3769       +4     
  Misses        794      794              
  Partials      209      209              

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[TwiN]

Thank you for your contribution!