feat: add SslResponse message

[elmaxxo]

Hello again! I'd like to introduce a new backend message.

The backend should communicate with the frontend using backend messages. But this simple concept is broken when you want to response to SslRequest messsage. There is no such message as SslResponse in postgres, so to answer to SslRequest they use a very special approach: the backend sends a single byte containing 'S' or 'N', indicating that it is willing or unwilling to perform SSL, respectively.

I don't like how this is done in postgres and, as for me, it is very unnatural, so this patch introduces SslResponse message, that simply wraps this single byte and provides the backend with convenient API for responding to an SslRequest message.

Also it is better in terms of symmetry: there is SslRequest in frontend messages, but there is no SslResponse in backend messages.

[sunng87]

No problem @elmaxxo . The reason why I didn't included SslResponse is I found it's weird too. I'm OK to have it in our codec, but you may want to update pgwire::tokio::peek_for_sslrequest as well to use the message type.

[elmaxxo]

Thank you for highlighting pgwire::tokio::peek_for_sslrequest. I have refactored it, please take a look.

I relized that there is no simple way to peek a message, as opposed to reading it. Don't you think that SslRequest should be handled in SimpleQueryHandler::on_startup? It would allow us to read such messages as well as Startup and handle them in place.

Also, shouldn't we handle CancelRequest?

[sunng87]

1. Good idea. Remember to keep a default implementation so that most developer won't need to know the detail of SSL handshake.
2. The whole cancel functionality is not implemented at the moment. It has a different pattern with default startup process. PR is welcomed but let me know your design before you get started.