feat: deprecate Headers for OrderedHeaders

[theseion]

Required for proper multi value header support in go-ftw (coreruleset/go-ftw#443).

[fzipi]

Why not making them always ordered?

[theseion]

That's the point. Headers is a map, so unordered and doesn't allow for duplicates. We need to keep Header for backwards compatibility though and I'm not aware of any mechanism that would allow us to easily accept both a list or a map for the same struct type.

[theseion]

@fzipi Anything you want to add?

[fzipi]

No, go ahead and merge.