feat(account): add channel_handle to AccountItem

[asomethings]

This PR adds channel_handle to AccountItem

Example response from latest AccountListEndpoint. It has new property called channelHandle.

{
  "contents": [
    {
      "accountItemSectionRenderer": {
        "contents": [
          {
            "accountItem": {
              "accountName": { "simpleText": "<Account Name>" },
              "accountPhoto": {
                "thumbnails": [
                  {
                    "url": "<URL>",
                    "width": 216,
                    "height": 216
                  }
                ]
              },
              "isSelected": true,
              "isDisabled": false,
              "hasChannel": true,
              "serviceEndpoint": {
                "selectActiveIdentityEndpoint": {
                  "supportedTokens": [
                    { "accountStateToken": { "hasChannel": true, "isMerged": true, "obfuscatedGaiaId": "<obfuscatedGaiaId>" } },
                    { "offlineCacheKeyToken": { "clientCacheKey": "<clientCacheKey>" } },
                    { "datasyncIdToken": { "datasyncIdToken": "<datasyncIdToken>" } }
                  ]
                }
              },
              "accountByline": { "simpleText": "<AccountByline>" },
              "channelHandle": { "simpleText": "@<Handle>" },
              "shouldShowPremiumLogo": false
            }
          }
        ]
      }
    }
  ]
}