gh-79156: Add start_tls() method to streams API (#91453)

The existing event loop `start_tls()` method is not sufficient for
connections using the streams API. The existing StreamReader works
because the new transport passes received data to the original protocol.
The StreamWriter must then write data to the new transport, and the
StreamReaderProtocol must be updated to close the new transport
correctly.

The new StreamWriter `start_tls()` updates itself and the reader
protocol to the new SSL transport.

Co-authored-by: Ian Good <icgood@gmail.com>