feat(alerts): add custom-alert command for user-defined alerts

[MicLieg]

Description

In this PR i've implemented a new ./gameserver custom-alert command.

This custom-alert command allows users to tailor LinuxGSM alerts to their precise needs and preferences, enabling more effective monitoring and communication of specific events, and enhancing the automation of tasks for increased efficiency.

When running the command ./gameserver custom-alert users will be prompted for a custom alert message:

The prompt can be skipped by running the command with a message as parameter ./gameserver custom-alert "Quick Custom Alert":

For even more customization a second parameter can be added to set a custom title./gameserver custom-alert "Quick Custom Alert" "Custom Title":

Fixes #4480

Type of change

• Bug fix (a change which fixes an issue).
• New feature (a change which adds functionality).
• New Server (new server added).
• Refactor (restructures existing code).
• Comment update (typo, spelling, explanation, examples, etc).

Checklist

PR will not be merged until all steps are complete.

• This pull request links to an issue.
• This pull request uses the develop branch as its base.
• This pull request subject follows the Conventional Commits standard.
• This code follows the style guidelines of this project.
• I have performed a self-review of my code.
• I have checked that this code is commented where required.
• I have provided a detailed enough description of this PR.
• I have checked if documentation needs updating.

Documentation

If documentation does need updating either update it by creating a PR (preferred) or request a documentation update.

• User docs: https://github.com/GameServerManagers/LinuxGSM-Docs
• Dev docs: https://github.com/GameServerManagers/LinuxGSM-Dev-Docs

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