Refactor ApiClient for better readability and performance

[GauravGavhane18]

Refactored ApiClient to use static final constants for configuration values like API_BASE_URL, connection timeout, and read timeout.

Improved response handling to avoid NullPointerException by adding proper null checks and handling both success and error responses.

Ensured exceptions are caught and returned as readable messages instead of crashing the application.

This change addresses the previous null pointer issue in ApiClient.

[cb-reshamsapkota]

While refreshing the oauth access token, if refresh token is already revoked, and if we do grant_type as refresh_token,
we get error object as:

    "error": {
        "description": "Token is already revoked"
    }
}

Here we don't have code key in error object, because of which we are getting RunTimeException instead of 4xx error.
I see this is being fixed in this PR.

private void throwException(int statusCode, JSONObject responseJson) throws RazorpayException {
        if (responseJson.has("error")) {
            JSONObject errorResponse = responseJson.getJSONObject("error");
            String code = errorResponse.getString("code");
            String description = errorResponse.getString("description");
            throw new RazorpayException(code + ":" + description);
        } else {
            this.throwServerException(statusCode, responseJson.toString());
        }
    }

But can we also include code for this error ? In that case, there would be no need for us to upgrade SDK.