Werk #16085: Agent Updater: Better detection of underlying platform

Previously, the agent updater used to detect the underlying platform, in terms of the combination of OS and package manager (e.g., Linux + RPM), automatically by scanning for certrain directories.

As multiple package managers may be available on one system, this sometimes lead to unexpected behavior on agent updates when the update mechanism decided to switch to a different platform.

Now, the platform will be specified by a file that comes with the agent installation, and the agent updater will rely on this static information instead of the dynamic detection.

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