Add support for id-mapped mounts to Exec directories (#34078)

Currently, bind-mounted directories within a user/mount namespace get
the uid/gid stored on their files. If the host creates a file in the
source directory, it will still show as root in the namespace.
Id-mapping is a filesystem feature that allows a mount namespace to show
a different uid than what is actually stored on a file. Add support for
id-mappings to exec directories, so that the files within the mount
namespace are owned by the unprivileged uid/gid.

Example:

Using unit:
```
[Unit]
Description=Sample service

[Service]
MountAPIVFS=yes
DynamicUser=yes
PrivateUsers=yes
TemporaryFileSystem=/run /var/opt /var/lib /vol
UMask=0000
ExecStart=/bin/bash -c 'while true; do echo "ping"; sleep 5; done'
StateDirectory=andresstatedir:sampleservice

[Install]
WantedBy=multi-user.target
```

In the host namespace, creating a file "test":
```
root@abeltran-test:/var/lib/andresstatedir# ls -lah
total 8.0K
drwxr-xr-x 2 root root 4.0K Aug 21 23:48 .
drwx------ 3 root root 4.0K Aug 21 23:47 ..
-rw-r--r-- 1 root root    0 Aug 21 23:48 test
```

Within the unit namespace:
```
root@abeltran-test:/var/lib/sampleservice# ls -lah
total 4.0K
drwxr-xr-x 2 63750 63750 4.0K Aug 21 23:48 .
drwxr-xr-x 3 root  root    60 Aug 21 23:47 ..
-rw-r--r-- 1 63750 63750    0 Aug 21 23:48 test
```
```
root@abeltran-test:/# mount | grep and
/dev/sda1 on /var/lib/private/andresstatedir type ext4 (rw,nosuid,noexec,relatime,idmapped,discard,errors=remount-ro,commit=30)
```