Fix annotation deserializing for the Wasi and Wcgi runners

[Michael-F-Bryan]

Here are a couple small fixes I added while making the wcgi-rust-template and wcgi-php-template work.

The key difference is that wapm2pirita will nest all annotations in your wasmer.toml file under an extra level (here).

That means, instead of a webc::metadata::CommandV2 looking like this

{
  "runner": "https://webc.org/runner/wcgi",
  "annotations": {
    "wcgi": {
      "dialect": "rfc-3875"
    }
  }
}

and being able to call command.get_annotations::<Wasi>("wasi"), the annotations actually look like this:

{
  "runner": "https://webc.org/runner/wcgi",
  "annotations": {
    "https://webc.org/runner/wcgi": {
      "wcgi": {
        "dialect": "rfc-3875"
      }
    }
  }
}

... and you need to create a temporary struct with the correct annotation types (e.g. webc::metadata::annotations::Wasi and webc::metadata::annotations::Wcgi in the WCGI case) so you can do let MyAnnotations { wcgi, wasi } = command.get_annotations(webc::metadata::annotations::WCGI_BASE_URI).

[ptitSeb]

Looks good to me, but I'm not very relevent in this subject.