Add Chat & Notify Get*Settings to use typed structs

Add Discord, Telegram, Mattermost, Matrix, and Google Chat integrations and use the v2.0 typed struct pattern.

Instructions

1. Investigation: Verify the properties JSON structure via the GitLab API.
2. Struct Definition: Implement dedicated structs (embedding Integration) and their Property structs for:
   • Discord
   • Telegram
   • Mattermost
   • Matrix
   • Google Chat
3. Refactor: Update method signatures in the IntegrationsService to return these new typed structs.
4. Validation: Update mocks and add relevant unit tests.

Change Need Implemented

• Discord: Add 

  DiscordIntegration, 

  DiscordIntegrationProperties

  , and 

  GetGroupDiscordSettings

  .

• Telegram: Add

  TelegramIntegration

  TelegramIntegrationProperties

  , and 

  GetGroupTelegramSettings

  .

• Mattermost: Add 

  MattermostIntegration

  MattermostIntegrationProperties

  , and 

  GetGroupMattermostSettings

  .

• Matrix: Add 

  MatrixIntegration

  MatrixIntegrationProperties

  , and 

  GetGroupMatrixSettings

  .

• Google Chat: Add 

  GoogleChatIntegration

  GoogleChatIntegrationProperties

  , and 

  GetGroupGoogleChatSettings

  .

Unlike the previous issues on this list, we are targeting the main branch!

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#2198