Fix PluginV1 fields backward compatibility

[smallinsky]

What

Fix feature UnmarshalPlugin compatibility

Extending plugin struct

When a new plugin or new field is added to the PluginV1 struct:

--- a/api/proto/teleport/legacy/types/types.proto
+++ b/api/proto/teleport/legacy/types/types.proto
@@ -5934,6 +5934,8 @@ message PluginOktaSettings {
+
+  bool EnableFeatureX = 5;
 }

the UnmarshalPlugin will marshal proto to the following JSON object:

{
  "kind": "plugin",
  "version": "v1",
   "spec": {
    "okta": {
       "enableFeatureX": true
    }
  },
  }
}

and the plugin will be successfully installed.

Downgrading Teleport Version

When Teleport is downgraded, the marshal function will fail with an unknown field error, producing a lot of periodic noise, and will not start the plugin, breaking backward compatibility.

This will also occurs when a new plugin type is added to PluginSpecV1

To fix that the AllowUnknownFields: true needs to be set.