Snap application cannot be run as snap_daemon user out of the box

[Shayan]

The MySQL charm uses the charmed-mysql snap (https://github.com/canonical/charmed-mysql-snap). This snap contains all the binaries required to run a MySQL charm (e.g. mysql-server, mysql-shell, xtrabackup, etc) installed from apt packages. The charmed-mysql snap has a daemon (https://github.com/canonical/charmed-mysql-snap/blob/8.0/edge/snap/snapcraft.yaml#L54-L60) to run mysqld, which is run as the snap_daemon user (https://github.com/canonical/charmed-mysql-snap/blob/8.0/edge/snap/local/start-mysqld.sh#L3-L4).

Since mysqld is run as snap_daemon, the MySQL data files are owned by the snap_daemon user. Thus, when the charm runs an application binary (e.g. xtrabackup - https://github.com/canonical/charmed-mysql-snap/blob/8.0/edge/snap/snapcraft.yaml#L100-L105), it must be run as snap_daemon to have access to the data files.

The home directory for the snap_daemon user is `/nonexistent` by default, and we run into the following error when we try to invoke the charmed-mysql.xtrabackup binary with the snap_daemon user in python using subprocess:

```
cmd_run.go:1046: WARNING: cannot create user data directory: cannot create snap home dir: mkdir /nonexistent: permission denied
cmd_run.go:1046: WARNING: cannot create user data directory: cannot create snap home dir: mkdir /nonexistent: permission denied
Sorry, home directories outside of /home are not currently supported.
See https://forum.snapcraft.io/t/11209 for details.
Sorry, home directories outside of /home are not currently supported.
```

The python code that tries to execute charmed-mysql.xtrabackup binary as the snap_daemon user is:

```
import subprocess

user = "snap_daemon"
group = "snap_daemon"
command = "/snap/bin/charmed-mysql.xtrabackup --help".split()
env = {}

process = subprocess.run(
    command,
    user=user,
    group=group,
    capture_output=True,
    text=True,
    env=env,
    encoding="utf-8",
)
print("stdout: ", process.stdout)
print("stderr: ", process.stderr)
```