Feature - Apollo Updater

[ItsNature]

Overview

Description: Add an updater that can be run with /apollo update

Changes: Add an updater that updates Apollo to the latest available version.

How it works:

1. Retrieve Apollo assets (Done only on server startup)
2. Locate the current jar file in use by Apollo (e.g. apollo-bukkit-1.0.8.jar).
3. Determine the download URL tailored to the platform in use.
4. Define the name & path for the updated jar file (e.g. /home/server/apollo-bukkit-1.0.9.jar).
5. Initiate the file download process using the DownloadFileRequest request.
6. If the download succeeds, the old Apollo jar file will be deleted during the server shutdown.

Add /apollo update command

• Permission: apollo.command

Add ApolloHttpManager#download(DownloadFileRequest) for file downloading

Review Request Checklist

• [x] Your code follows the style guidelines of this project.
• [x] I have performed a self-review of my code.
• [x] I have tested this change myself. (If applicable)
• [x] I have made corresponding changes to the documentation. (If applicable)
• [x] The branch name follows the projects naming conventions. (e.g. feature/add-module & bugfix/fix-issue)

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