Headers announcement for nodes that can do headers. #9721

This was raised previously (#8961) but closed due to an apparent misunderstanding - it builds upon the current announce by headers logic in that it will also detect nodes that understand headers and announce via headers to these also (I am not aware of any advantage in announcing via block invs to nodes that actively request headers and therefore can process them).

@rebroad BIP 130 has already been implemented and allows peers to opt-in to receiving block announcements via headers, specifically to maintain backwards compatibility and not require all software on the network adopt the protocol change. This patch only makes the behavior more confusing (as it needlessly breaks BIP 130's opt-in protocol) and less flexible (as peers which make use of getheaders are now forced to receive block announcements this way as well, which could break other participants on the network). This is why I NACKed this when you proposed it in #8961.

As an aside, I do think we could propose eliminating block inv's altogether, which has been suggested before. But that is a topic that should first be discussed more broadly, eg on the bitcoin-dev mailing list, as it would affect all software on the peer-to-peer network.