fshttp: add --dump trace to log connection level events via httptrace

The new "trace" dump flag attaches a net/http/httptrace ClientTrace to
each HTTP transaction and logs the connection level events - DNS
resolution, TCP connect, TLS handshake (including the negotiated TLS
version, cipher, ALPN protocol and server certificate), connection
reuse, request write and time to first response byte. Each line is
tagged with the time elapsed since the start of the transaction and the
request pointer so it can be correlated with the other dumps.

This is complementary to the existing dump flags: it shows how the
connection behaved rather than what was sent, which is useful for
debugging connectivity, DNS, TLS, proxy and keep-alive problems.

docs/content/docs.md

@@ -3296,6 +3296,15 @@ The available flags are:
   failed transactions only. On its own `--dump errors` dumps the headers.
   This lets you capture first-failure diagnostics without the noise of
   dumping every transaction. May contain sensitive info.
+- `trace` logs connection level events for each HTTP transaction using Go's
+  `net/http/httptrace` - DNS resolution, TCP connect, TLS handshake (including
+  the negotiated TLS version, cipher, ALPN protocol and server certificate),
+  connection reuse, request write and time to first response byte. Each line is
+  tagged with the time elapsed since the start of the transaction so you can see
+  where the time goes. This is complementary to the other dump flags: it shows
+  how the connection behaved rather than what was sent, so it is useful for
+  debugging connectivity, DNS, TLS, proxy and keep-alive problems. It does not
+  dump headers or bodies - combine it with `headers` or `bodies` for that.
 
 ## Filtering