Use recursive generics starting from go1.26

Given https://x.com/i/status/2009244177960251864 we can fix https://github.com/celestiaorg/go-header/blob/main/header.go#L9-L11

Start with header.go lines 9–11 and read the linked discussion to understand the proposed recursive generics change for Go 1.26. Review the repository's current type definitions and run its existing checks; done means the intended generic structure is implemented without breaking the library's current behavior.

Written by the indexing model from the issue text.