Enable ZenFS functionality on standard Percona Server packages on Debian 11 and Ubuntu 20.04 Focal

At the moment Debian 11.1 is the only platform that meets all ZenFS requirements out-of-the-box without any modifications.
By default it has 5.10.70 LTS kernel that includes CAP_SYS_ADMIN patch in block/blk-zoned.c.
At the same time, although default LTS kernel on Ubuntu 20.04 Focal is of version 5.4.0, a 5.11 HWE kernel patched with CAP_SYS_ADMIN can be pretty easily installed on that platform. Moreover, with this patch in libzbd code, the library can now be built even when linux-libc-dev is of version 5.4.0 and does not have BLKOPENZONE / BLKCLOSEZONE / BLKFINISHZONE constants defined in <linux/blkzoned.h>
Therefore, for Debian 11 and Ubuntu 20.04 Focal

1. Percona Server must be built with -DROCKSDB_PLUGINS=zenfs, -DWITH_ZENFS_UTILITY=ON and -DWITH_ZBD=bundled CMake options
2. zenfs utility must be added to the package binaries list
3. libgflags must be added as package dependenciy
4. libgflags-dev must be added as package build dependency

Note: WITH_ZBD CMake option was added in this commit in percona-server and is currently available in 8.0 trunk.