feat(alerting): Add group-specific WebHook URL for Discord

[appleboy]

Add group-specific webhook URL for discord alert

Provides support for paging multiple Discords based on the group selector while keeping backward compatibility to the old Discords configuration manifest

integration per team can be specified in the overrides sections in an array form.

alerting:
  discord: 
    webhook-url: "https://discord.com/api/webhooks/**********/**********"
    # You can also add group-specific to keys, which will 
    # override the to key above for the specified groups
    overrides:
      - group: "core"
        webhook-url: "https://discord.com/api/webhooks/************"

endpoints:
  - name: website
    url: "https://twin.sh/health"
    interval: 30s
    conditions:
      - "[STATUS] == 200"
      - "[BODY].status == UP"
      - "[RESPONSE_TIME] < 300"
    alerts:
      - type: discord
        enabled: true
        description: "healthcheck failed"
        send-on-resolved: true

  - name: back-end
    group: core
    url: "https://example.org/"
    interval: 5m
    conditions:
      - "[STATUS] == 200"
      - "[CERTIFICATE_EXPIRATION] > 48h"
    alerts:
      - type: discord
        enabled: true
        description: "healthcheck failed"
        send-on-resolved: true

ref: #96

Signed-off-by: Bo-Yi Wu appleboy.tw@gmail.com

[appleboy]

@TwiN All Done, Please help to review again.

[appleboy]

@TwiN All Done.

[codecov-commenter]

Codecov Report

Merging #271 (60205c1) into master (5281f80) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #271      +/-   ##
==========================================
- Coverage   83.70%   83.68%   -0.03%     
==========================================
  Files          53       53              
  Lines        3462     3476      +14     
==========================================
+ Hits         2898     2909      +11     
- Misses        439      441       +2     
- Partials      125      126       +1     

Impacted Files	Coverage Δ
alerting/provider/discord/discord.go	91.78% <100.00%> (+1.95%)	⬆️
config/maintenance/maintenance.go	92.40% <0.00%> (-3.80%)	⬇️

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[appleboy]

@TwiN Done. Please review again.

[TwiN]

My bad, I missed the notification.
Thank you for the contribution!