LXD supports using --profile multiple times, but the plugin does not

#### Description

LXD supports using --profile multiple times:
`lxc launch --profile --profile ...`

The plugin does, however, only support a single profile given as a string.

#### References

- https://linuxcontainers.org/lxd/docs/master/profiles/#apply-a-profile-to-an-instance
- https://github.com/hashicorp/packer-plugin-lxd/blob/main/builder/lxd/config.go#L33
- https://github.com/hashicorp/packer-plugin-lxd/blob/main/builder/lxd/step_lxd_launch.go#L22

Start with builder/lxd/config.go and builder/lxd/step_lxd_launch.go, then trace how the profile value reaches the launch command. Update the configuration and launch handling so multiple --profile values are supported, and verify that each supplied profile is passed to LXD.

Written by the indexing model from the issue text.