Add support for closing a database (session)

It seems that Apache Arrow Flight SQL doesn't provide a command that closes the current session explicitly:
https://arrow.apache.org/docs/format/FlightSql.html

In https://lists.apache.org/thread/0w25o85y8vsndz87kpjljxz24x077o3y , using a custom (close) action was suggested.

@lidavidm Is using a custom close action still a suggested approach to close the current session? I have a concern about this approach. If each Apache Arrow Flight SQL server implementation uses different custom close action, users can't use a general Apache Arrow Flight SQL client including ADBC to use Apache Arrow Flight SQL servers. (Users need to implement a custom close action for each Apache Arrow Flight SQL server.)

先查看 Apache Arrow Flight SQL 格式页面以及其中链接的邮件列表讨论串，以确定标准会话关闭机制的状态。该 issue 没有指定源文件或测试；要完成这项工作，需要一个达成共识且可互操作的方法，以及针对该 PostgreSQL 适配器的具体实现范围。

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