Add support for TLVs

[emersion]

The PP2_TYPE_ALPN TLV is necessary to negotiate HTTP/2.

[mastercactapus]

I started this in the tlv branch, and want to test it with some in-the-wild software before merging & releasing.

[vmihailenco]

@mastercactapus I am about to try this so I wonder what were your conclusions after testing TLV support?

[Embraser01]

I started this in the tlv branch, and want to test it with some in-the-wild software before merging & releasing.

Hi @mastercactapus, after having issue with AWS Network Load Balancer (that add TLV), I tested your tlv branch with caddy v2.7.4 and I'm happy to say that it worked perfectly 💯

Would it be possible to release a new version with the TLV feature?

PS : For those who want to test it with Caddy, you can do it like this:

xcaddy build \
  --with github.com/mastercactapus/proxyprotocol=github.com/mastercactapus/proxyprotocol@tlv