adapter: support custom or empty device port name prefixes

This MR adds support for setting a custom or empty port name prefix for device ports. Other ports (like monitor) are not affected.

The custom prefix can be set via the node.device-port-name-prefix property. Passing an empty string disables the prefix completely.

Sample ALSA monitor rule for a custom prefix:

rename_2i2_input_ports = {
  matches = {
    {
      { "node.name", "equals", "alsa_input.usb-Focusrite_Scarlett_2i2_USB-00.analog-stereo" },
    },
  },
  apply_properties = {
    ["node.device-port-name-prefix"] = "hello-world",
    ["node.channel-names"] = '[front1,front2]',
  }
}

rename_2i2_output_ports = {
  matches = {
    {
      { "node.name", "equals", "alsa_output.usb-Focusrite_Scarlett_2i2_USB-00.analog-stereo" },
    },
  },
  apply_properties = {
    ["node.device-port-name-prefix"] = "hello-world",
    ["node.channel-names"] = '[speaker_left,speaker_right]',
  }
}

table.insert(alsa_monitor.rules, rename_2i2_input_ports)
table.insert(alsa_monitor.rules, rename_2i2_output_ports)

Screenshot from Helvum

Sample ALSA monitor rule for disabled prefix:

rename_2i2_input_ports = {
  matches = {
    {
      { "node.name", "equals", "alsa_input.usb-Focusrite_Scarlett_2i2_USB-00.analog-stereo" },
    },
  },
  apply_properties = {
    ["node.device-port-name-prefix"] = "",
    ["node.channel-names"] = '[front1,front2]',
  }
}

rename_2i2_output_ports = {
  matches = {
    {
      { "node.name", "equals", "alsa_output.usb-Focusrite_Scarlett_2i2_USB-00.analog-stereo" },
    },
  },
  apply_properties = {
    ["node.device-port-name-prefix"] = "",
    ["node.channel-names"] = '[speaker_left,speaker_right]',
  }
}

table.insert(alsa_monitor.rules, rename_2i2_input_ports)
table.insert(alsa_monitor.rules, rename_2i2_output_ports)

Screenshot from Helvum