fix: treat BusinessTeam as a regular Team for Environment reviewers

[JWilkinsonMB]

It is now supported to use Enterprise Teams as reviewers for Environments, however the type returned by the API has changed, so that instead of Team, it is now BusinessTeam. Sample JSON below:

{
  "reviewers": [
    {
      "type": "BusinessTeam",
      "reviewer": {
        "name": "Name",
        "id": 123456,
        "node_id": "ET_xyz123456",
        "slug": "ent:xyz123",
        "description": "",
        "privacy": "closed",
        "notification_setting": "notifications_enabled",
        "url": "https://api.github.com/enterprises/xyz/teams/123456",
        "html_url": "https://github.com/enterprises/xyz/teams/ent:desktop",
        "members_url": "https://api.github.com/enterprises/xyz/teams/123456/memberships{/member}",
        "repositories_url": "https://api.github.com/enterprises/xyz/teams/123456/repos",
        "type": "enterprise",
        "enterprise_id": 123,
        "permission": "pull",
        "parent": null
      }
    }
  ]
}

Prior to this PR this leads to an unmarshaling error if an enterprise team is included as a reviewer in any environment that the library tries to read.

Whilst there are some differences between a BusinessTeam and a Team such as enterprise_id instead of organization_id, a quick workaround is to treat both types as a Team given there is sufficient overlap which allows consumers such as the Terraform provider to be able to work without any breaking changes.

Fixes #4457

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