Add an `sni_hostname` request extension.

[tomchristie]

Add a request extension called "sni_hostname", which may be used to control the hostname used when initiating an SSL connection.

This would be an optional override, instead of using the hostname from the URL...

httpcore/httpcore/_async/connection.py

Lines 135 to 142 in 5f2e454

	kwargs = {
	"ssl_context": ssl_context,
	"server_hostname": self._origin.host.decode("ascii"),
	"timeout": timeout,
	}
	async with Trace("connection.start_tls", request, kwargs) as trace:
	stream = await stream.start_tls(**kwargs)
	trace.return_value = stream

Refs: encode/httpx#2592 (comment)