chore(slogutil): add configurable logging format (fixes #10352) (#10354)

This adds several options for configuring the log format of timestamps
and severity levels, making it more suitable for integration with log
systems like systemd.

      --log-format-timestamp="2006-01-02 15:04:05"
         Format for timestamp, set to empty to disable timestamps ($STLOGFORMATTIMESTAMP)

      --[no-]log-format-level-string
         Whether to include level string in log line ($STLOGFORMATLEVELSTRING)

      --[no-]log-format-level-syslog
         Whether to include level as syslog prefix in log line ($STLOGFORMATLEVELSYSLOG)

So, to get a timestamp suitable for systemd (syslog prefix, no level
string, no timestamp) we can pass `--log-format-timestamp=""
--no-log-format-level-string --log-format-level-syslog` or,
equivalently, set `STLOGFORMATTIMESTAMP="" STLOGFORMATLEVELSTRING=false
STLOGFORMATLEVELSYSLOG=true`.

Signed-off-by: Jakob Borg <jakob@kastelo.net>

internal/slogutil/formatting_test.go

@@ -17,8 +17,11 @@ import (
 func TestFormattingHandler(t *testing.T) {
 	buf := new(bytes.Buffer)
 	h := &formattingHandler{
-		out:          buf,
-		timeOverride: time.Unix(1234567890, 0).In(time.UTC),
+		opts: &formattingOptions{
+			LineFormat:   DefaultLineFormat,
+			out:          buf,
+			timeOverride: time.Unix(1234567890, 0).In(time.UTC),
+		},
 	}
 
 	l := slog.New(h).With("a", "a")