Implement BetaPluginSource to fetch plugins from server

[mleem97]

• Replace hardcoded empty URL with retrieving URL from S.Preferences.
• Remove OS-specific #if block.
• Replace GetAsync().GetAwaiter().GetResult() and ReadAsStringAsync().GetAwaiter().GetResult() with synchronous Send() and ReadAsStream().
• Add test BetaPluginSourceTests.

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PR created automatically by Jules for task 15673601384401466055 started by @mleem97

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