Allow specifying mutability in avocadoctl config

[mobileoverlord]

Current behavior is to merge /usr /opt and /etc with an ephemeral upper r/w layer. In this configuration, runtime changes to these directories will be lost after a reboot. This PR allows configuring the --mutable= value from avocado config

# /etc/avocado/avocadoctl.conf

# Example avocadoctl configuration file
# This file demonstrates the available configuration options

[avocado.ext]
# Directory where extensions are stored
dir = "/var/lib/avocado/extensions"

# Mutability mode for extensions (systemd --mutable option)
# Valid values: no, auto, yes, import, ephemeral, ephemeral-import
# Default: ephemeral
mutable = "ephemeral"

# Examples of other valid values:
# mutable = "no"           # Force immutable mode
# mutable = "auto"         # Enable mutable mode if write routing directories present
# mutable = "yes"          # Force mutable mode, create write routing directories
# mutable = "import"       # Immutable mode but merge write routing contents
# mutable = "ephemeral-import" # Mutable mode with write routing merged but changes discarded after unmerge

More information on mutable at the systemd docs