Get database size and available capacity

Add ways to retrieve information about sizes, e.g.:

• database size on disk
• current size limit on disk
• information about free space in the current file. This is not trivial and could be e.g. based on free pages and pages with capacity. Might be an approximation. Could be a percentage or an absolute number.
• optional(?): approximate remaining capacity until we reach max size (and/or disk limit). Details tbd...

未指定文件、测试或入口点。首先检查现有的数据库 API，以及存储限制和可用空间的表示方式，然后确定需要哪些度量，以及可用空间是否可以是近似值。完成的标准是：以约定的语义公开所选的大小和容量信息，并由测试覆盖。