Support for transparent SNI proxying

This is more of a PR than an issue, since I've implemented the code already, but it's in a separate server:

https://code.betamike.com/micropelago/tokio-rustls/commit/18fd688b335430e17e054e15ff7d6ce073db2419

Implement TransparentConfigAcceptor

The goal of the TransparentConfigAcceptor is to support an SNI-based
reverse-proxy, where the server reads the SNI and then transparently
forwards the entire TLS session, ClientHello included, to a backend
server, without terminating the TLS session itself.

This isn't possible with the current LazyConfigAcceptor, which only
allows you to pick a different ServerConfig depending on the SNI, but
will always terminate the session.

The TransparentConfigAcceptor will buffer all bytes read from the
connection (the ClientHello) internally, and then replay them if the
user decides they want to hijack the connection.

The TransparentConfigAcceptor supports all functionality that the
LazyConfigAcceptor does, but due to the internal buffering of the
ClientHello I did not want to add it to the LazyConfigAcceptor, since
it's possible someone wouldn't want to incur that extra cost.

I'm very much open to feedback on these changes. I'm relatively new to rust, and would not be surprised if I overcomplicated this a bit. In particular the types I added to common seem like stuff that should already exist in some standard crate somewhere, but I couldn't find them, and probably my implementation is lacking. Also the API I've introduced generally seems kind of ugly to me... there's probably some way to clean it up some.

Start by reviewing the linked external commit and the existing LazyConfigAcceptor to understand the proposed TransparentConfigAcceptor API and buffering behavior. Done means supporting SNI-based transparent forwarding of the complete TLS session, including ClientHello, without terminating it, with the API and common types reviewed for this project.