sql: augment statement logging with a txn counter

[jseldess]

PR: cockroachdb/cockroach#41929

From release notes:

The structured payloads used for SQL audit and execution logs now include a transaction counter since the beginning of the session. Statements issued inside the same SQL transaction will thus be logged with the same counter value, thus enabling per-transactions grouping during log analysis. Separate sessions use independent counters. [#41929][#41929] {% comment %}doc{% endcomment %}

[jseldess]

cc @ericharmeling, @ianjevans, this might be SQL observability, not sure.

[taroface]

This is about the log messages, so it should be my area.

[taroface]

Closed by #10273.