Previously the plugin installer attempted to select the correct plugin
binary by its file extension but it turns out that this is not a unique
choice (e.g. .so is supported on macOS and on Linux).
Therefore this commit introduces a mandatory manifest file that is to be
present in plugin bundles that contains the mapping for which binary to
use on which OS and for which architecture.
Because a plugin bundle now has to follow such a strict format, it is
now also mandatory for the bundle to have the file extension
.mumble_plugin (previously .zip was allowed as well).
