sql: non-historical statement_timestamp with AOST

When using historical queries with AS OF SYSTEM TIME, functions that return the transaction timestamp such as `now` and `current_timestamp` and `transaction_timestamp` are set to the historical time. But this is not true for `statement_timestamp` which is set to the current wall time.

According to [PG docs](https://www.postgresql.org/docs/current/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT):

> statement_timestamp() and transaction_timestamp() return the same value during the first command of a transaction, but might differ during subsequent commands.

In order for this to be true, `statement_timestamp` should also be set to the historical time.

Jira issue: CRDB-51068