mariadb-backup compatible wrappers for BACKUP SERVER

The server-side BACKUP SERVER statement (MDEV-14992) is a fundamentally different way of performing backups, compared to the current mariadb-backup tool. Likewise, streaming backup (MDEV-38362) will necessarily be quite different from what we have now.

While many concepts are different, it should be possible to implement a partial compatibility layer for the mariadb-backup and mbstream tools, so that existing scripts around backup can continue to work with minimal changes.

The mariadb-backup compatible tool would invoke either BACKUP SERVER in a client connection for backup operation, or mariadbd itself for prepare operation. Some operations, such as copy_back or move_back, could be implemented by invoking mysys functions. The tool would also have to examine the metadata in the specified directory.

This will have to be designed in detail once MDEV-14992 has been completed.

Designing and implementing support for streaming operations is obviously blocked by MDEV-38362. The mbstream compatible tool would be a wrapper for tar x, assuming that we adopt the POSIX tar format.