Add %transport::>connection_id logformat code (#546)

Add %transport::>connection_id logformat code (#546)

Knowing client transport connection ID can be very helpful in triage,
especially when dealing with multiple requests associated with a
problematic connection (e.g., two access log records associated with a
single TLS connection bumping failure).

The "transport" namespace was added in hope to reduce confusion when/if
other connection_id %codes are added. The shorter and more familiar
"tcp::" namespace was rejected because we want configurations using the
new %code to continue to work well after Squid gains HTTP/3 support, and
HTTP/3 uses UDP for transport connections. Other UDP-based protocols may
have similar "connection" concepts.

This is a Measurement Factory project.