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.)

先從 Apache Arrow Flight SQL 格式頁面以及其中連結的郵件列表討論串開始，以確定標準工作階段關閉機制的狀態。該 issue 沒有指定原始碼檔案或測試；要完成這項工作，需要一個達成共識且可互通的方法，以及針對此 PostgreSQL adapter 的具體實作範圍。

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