Function validate_http_sink 

fn validate_http_sink(
    is_append_only: bool,
    ignore_delete: bool,
    schema: &Schema,
    url: Option<&str>,
    content_type: Option<&str>,
    headers: &BTreeMap<String, String>,
) -> Result<HttpSink>

Validates the HTTP sink parameters and returns the sink so callers can use it directly without re-parsing.