Improve the header.IPv6 API

**Description**

The IPv6 Header api follows the wording of RFC 8200: the extension headers are considered payload, and the payload length takes into account the extension headers.

It might make sense to change the API to consider that a payload is strictly the upper-layer payload.

**Do you have a specific solution in mind?**

We could change the API so that
Payload() -> only returns the upper-layer payload
PayloadLength() -> only returns the length of the upper-layer payload
Then add functions to expose information about the extension headers
ExtensionHeaders() -> returns the extension headers only
ExtensionHeadersLength() -> returns the extension headers length
And probably some way to return both payloads at once

Note: we need to parse the extension header to find out the length of the payloads, so it might be a good idea to store the lengths in separate fields - however header.IPv6 is written directly into the packet buffers so we can't store extra data in it. Maybe create a new type that wraps header.IPv6 and adds the new meta data next to it.