fix(core): Add channels to asset entities on admin api

[DanielBiegler]

Description

I found that the channel-aware asset entities obviously have a channels field but its actually not exposed in the admin api. It got added in #700 but cant be queried.

Please note: I purposefully have not included re-generated types to make this PR as concise and review-able as possible.

For finishing this PR it should probably include this newly exposed field in the e2e-tests! Note that you'll need to look into the following when running:

mutation assign {
  assignAssetsToChannel(input: { assetIds: ["50"], channelId: "2" }) {
    name
    channels {
      id
      code
    }
  }
}

You'll see a

{
  "errors": [
    {
      "message": "Cannot return null for non-nullable field Asset.channels.",
      "locations": [
        {
          "line": 37,
          "column": 5
        }
      ],
      "path": [
        "assignAssetsToChannel",
        0,
        "channels"
      ]
    }
  ],
  "data": null
}

eventhough the mutation will go through and work!

{
  "data": {
    "asset": {
      "name": "kari-shea-398668-unsplash.jpg",
      "channels": [
        {
          "id": "1",
          "token": "pxmu1fra2nrky6xivt",
          "code": "__default_channel__"
        },
        {
          "id": "2",
          "token": "example",
          "code": "Example"
        }
      ]
    }
  }
}

Please someone feel free to take over this PR, I just wanted to create this as the starting point for fixing #2478 but theres more to do there for example maybe a removeAssetsFromChannel mutation and ui which I dont want to do because of Angular & RxJs 😄

Breaking changes

Should be none AFAIK

Screenshots

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Checklist

📌 Always:

• I have set a clear title
• My PR is small and contains a single feature
• I have checked my own PR

👍 Most of the time:

• I have added or updated test cases
• I have updated the README if needed

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