Return direct connection details

What does this MR do and why?

Adds an API endpoint which can be used by FE to get all necessary info about direct connection details (url, token) which it should use for sending code completions requests.

Related to #452044 (closed)

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Before	After

How to set up and validate locally

1. Feature.enable(:code_suggestions_direct_completions)
2. Test that the expected response is returned:

curl --header "PRIVATE-TOKEN: $TOKEN" --request POST http://192.168.1.8:3000/api/v4/code_suggestions/direct_access|jq .

should return a response similar to:

{
      "base_url": "http://0.0.0.0:5052",
      "token": "eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiJ9.eyJqdGkiOiI3YWFmYzA0Zi04MDMyLTQ1NDctYWRiZC0xNDkxMDg0NzUyMTkiLCJhdWQiOiJnaXRsYWItYWktZ2F0ZXdheSIsInN1YiI6IjI5MmMzYzdjLWM1ZDUtNDhlYy1iNGJmLWYwMGI3MjRjZTU2MCIsImlzcyI6Imh0dHA6Ly8xOTIuMTY4LjEuODozMDAwIiwiaWF0IjoxNzEzMjY0NTY2LCJuYmYiOjE3MTMyNjQ1NjEsImV4cCI6MTcxMzI2ODE2NiwiZ2l0bGFiX3JlYWxtIjoic2FhcyIsInNjb3BlcyI6WyJjb2RlX3N1Z2dlc3Rpb25zIl19.G7tacEBx9gigUjpkgLkas_yGveioc7TgBrH6d2lTcF4VR61GxRJ6zCgMEz-ZRbKaRWV7yMKlXdVO_TGd_MYpnB9Bs331N-LUpXcgdj5KaWE8f5PhVLnISRcgF8GnlMzovVGJyrhBSmsNBbO8c-7Rgw6onjEW-8zQTswbQrDENneIZMoEoCdaIFQLtkvRaxbcokXvkD8RHgKYEMwp1Hkzca9qeJB83i2SIeJ08PhToh1o13KsDIaHUDTOnGPCu3d-DzZSM5KTbM_yHW8rpTXqvPhvxqhckPEayTgPGVd6jqp96zWqrzZKtMQr_4sTuLx2bijADxK_ltY_tvnv73NEXg",
      "expires_at": 1713343569,
      "headers": {
        "X-Gitlab-Instance-Id": "292c3c7c-c5d5-48ec-b4bf-f00b724ce560",
        "X-Gitlab-Realm": "saas",
        "X-Gitlab-Global-User-Id": "Df0Jhs9xlbetQR8YoZCKDZJflhxO0ZBI8uoRzmpnd1w=",
        "X-Gitlab-Host-Name": "192.168.1.8"
      }
}