Add OAuth identification

[prusfr]

Add OAuth identification as per the opensky new authentication strategy.

I added the following snippets

1. New method of OpenSkyApi class :
`def _get_opensky_token(self, client_id:str, client_secret:str):
"""
Retrieves an access token from the OpenSky Network API.

    This function sends a POST request to the OpenSky Network's token endpoint
    to obtain a bearer token for API access. 
    Returns:
        str: The access token if the request is successful, otherwise None.
    """

    if not client_id or not client_secret:
        print("Error: OPENSKY_CLIENT_ID and OPENSKY_CLIENT_SECRET environment variables must be set.")
        return None

    # The URL for the token endpoint
    token_url = "https://auth.opensky-network.org/auth/realms/opensky-network/protocol/openid-connect/token"

    # The data payload for the POST request
    payload = {
        "grant_type": "client_credentials",
        "client_id": client_id,
        "client_secret": client_secret,
    }

    # The headers for the request
    headers = {
        "Content-Type": "application/x-www-form-urlencoded"
    }

    try:
        # Make the POST request
        response = requests.post(token_url, headers=headers, data=payload)

        # Raise an exception for bad status codes (4xx or 5xx)
        response.raise_for_status()

        # Parse the JSON response and extract the access token
        access_token = response.json().get("access_token")
        return access_token

    except requests.exceptions.RequestException as e:
        print(f"An error occurred during the request: {e}")
        # If the response has content, print it for more details
        if e.response:
            print(f"Response content: {e.response.text}")
        return None
    except ValueError:
        # Catches JSON decoding errors
        print("Error: Failed to decode JSON response.")
        print(f"Response content: {response.text}")
        return None`

2. Changes in __init__ constructor:

Add the following code at the end of the __init__ method:
self._headers = { "Authorization": f"Bearer {self._get_opensky_token(client_id=username, client_secret=password)}" }

3. Change in _get_json method:
Change the line auth=self._auth with headers=self._headers.