[settings] Add configurable default image

[Jotanune]

Closes #2653

Summary

Add a new local.default-image setting that allows users to configure which image is launched by default when no image is specified in multipass launch.

Problem

Currently, multipass launch without an image argument always defaults to the latest Ubuntu LTS (via the hardcoded "default" alias). There is no way to change this behavior.

Solution

• Declare a new local.default-image setting key in constants.h
• Register it as a BasicSettingSpec with an empty default value in the daemon
• Read the setting in query_from() and make_cloud_init_vendor_config() to use the configured image when no image is specified in a launch request
• Fall back to "default" (latest LTS) when the setting is empty

Usage

multipass set local.default-image=22.04
multipass launch          # launches 22.04 instead of the latest LTS
multipass launch 24.04    # explicit image still takes precedence
multipass set local.default-image=   # reset to default behavior

Testing

• Updated daemonRegistersPersistentHandlerForDaemonSettings to verify the new setting is registered with an empty default value
• All 16 TestGlobalSettingsHandlers tests pass

[Jotanune]

This PR currently includes the commit from my other PR (#4792) because I needed that fix to be able to compile and test this feature locally on Windows. Once #4792 is merged into main, GitHub will automatically hide that commit from this PR. I can also rebase if you prefer.