Adding a special callback for the `HTTP2-Settings` header value for clients and servers that can upgrade to HTTP/2

I think it would be a good idea if there was a span callback specifically for `HTTP2-Settings` where it's base64 value can be streamed separately optionally with a flag for `llhttp_t` Where it could be something that could concurrently be streamed alongside the other callbacks. I could add restrictions to the parser in the strict mode to also ensure that if `llhttp_t` has a flag for upgrading to HTTP2 that the parser has an added check to ensure that HTTP2-Settings's value uses valid url-safe base64 characters.