Add BEGIN/END, BEGIN..WITH support

[seancorfield]

Per Jeremy @refset

You can set the system time of a transaction as part of the BEGIN statement - BEGIN READ WRITE WITH (system_time = TIMESTAMP '...') (or BEGIN READ WRITE WITH (system_time = ?) if you want to pass it as a parameter).

[seancorfield]

Examples of this syntax in various databases:

• MS SQL Server https://www.sqlservertutorial.net/sql-server-stored-procedures/sql-server-begin-end/
• https://www.sqltutorial.net/begin-transaction.html
• https://learn.microsoft.com/en-us/sql/t-sql/language-elements/begin-transaction-transact-sql?view=sql-server-ver17
• XTDB https://docs.xtdb.com/reference/main/sql/txs.html
• https://docs.xtdb.com/about/txs-in-xtdb.html
• https://docs.xtdb.com/reference/main/sql/queries#basis