IPv6 endpoint MaxHeaderLength doesn't consider extension headers

The common pattern to build frames at the transport layer is to allocate a buffer of sufficient size based on the response from `Route.MaxHeaderLength()`, which will in turn probe the `NetworkEndpoint` for its `MaxHeaderLength` implementation.

The IPv6 implementation currently lies about the maximum header length, adding only the minimum IPv6 header size to it, in order to prevent huge unused allocations.

Once `NetworkHeaderParams` introduces parameters that will be encoded as extension headers, upper layers will need a way to reserve the appropriate buffer space, which is likely dependent on the value of `NetworkHeaderParams`.

Perhaps `Route.MaxHeaderLength` should take `NetworkHeaderParams` as an argument.