tcptls/iostream: Add support for setting SNI on client TLS connections

tcptls/iostream:  Add support for setting SNI on client TLS connections

If the hostname field of the ast_tcptls_session_args structure is
set (which it is for websocket client connections), that hostname
will now automatically be used in an SNI TLS extension in the client
hello.

Resolves: #713

UserNote: Secure websocket client connections now send SNI in
the TLS client hello.

(cherry picked from commit 9e1a6fa0a741b02182245b56c8f78e489f25ed54)