Refactor feature extensions out of VMManager

[joshua-kim]

Why this should be merged

Currently, all feature extension code is registered as a VM (even though they aren't actually VMs) under the VMManager. This leads to strange hacks like returning the opaque any type since we can return either a vm factory or an fx factory, which leads to us also having to perform more type casting to sanity check that we're actually using a vm instead of a potential fx.

This PR hard-codes the fxs that we're using to just be a map, effectively deprecating plugin feature extensions that aren't used anyways. Only the AVM as far as I am aware is using these.

How this was tested

CI passes. Calling the GetVMs api additionally now only returns vms, instead of previously returning the feature extensions as well.

➜  subnet-evm git:(master) curl -sX POST --data '{
    "jsonrpc":"2.0",
    "id"     :1,
    "method" :"info.getVMs",
    "params" :{}
}' -H 'content-type:application/json;' 127.0.0.1:9650/ext/info  | jq
{
  "jsonrpc": "2.0",
  "result": {
    "vms": {
      "jvYyfQTxGMJLuGWa55kdP2p2zSUYsQ5Raupu4TW34ZAUBAbtq": [
        "avm"
      ],
      "mgj786NP7uDwBCcq6YwThhaN8FLyybkCa4zBWTQbNgmK6k9A6": [
        "evm"
      ],
      "rWhpuQPF1kb72esV2momhMuTYGkEb1oL29pt2EBXWmSy4kxnT": [
        "platform"
      ]
    }
  },
  "id": 1
}

[joshua-kim]

Another idea is to return an fx.Interface interface here, but we're already currently returning the any type so I'm going to punt this off to a follow-up since my focus is to currently just refactor the vm code.