support HTTP/1.1 vs HTTP/2 protocol negotiation for client connections

Aim would be to request a HTTP/2 client connection but to allow failover to HTTP/1.1 if the server-side does not support HTTP/2.

See https://pekko.apache.org/docs/pekko-http/current/client-side/http2.html#http-2-over-tls - which says:

Apache Pekko HTTP does not currently support protocol negotiation to fall back to HTTP/1.1 for this API. When the server does not support HTTP/2, the stream will fail.

Start with the HTTP/2 client-side documentation linked in the issue, especially the HTTP/2 over TLS section, and trace the client connection API it describes. Define completion as negotiating HTTP/2 when supported and falling back to HTTP/1.1 when the server does not support HTTP/2, without the stream failing.

Written by the indexing model from the issue text.