Select vendor boot debug

[bridadan]

vendor_boot.img can be replaced with vendor_boot-debug.img to enable
root on user builds. This is required for certain tests. This is already
possible when using the cvd create --vendor_boot_image=path/to/vendor_boot-debug.img flag, however it isn't
possible to do this when using config files
(cvd create --config_file config.json).

This change adds a use_vendor_boot_debug field to the config files to
allow fetching (if necessar) and then using the vendor_boot-debug.img
for a build. It also works with local builds.

It is also now possible to specify --use_vendor_boot_debug when
calling cvd create. If this is set when --vendor_boot_image is also
set, --use_vendor_boot_debug is ignored.

Bug: 542722431
Test: Create a file named config.json with the follwing format:

    {
      "common": {
        "host_package": "@ab/git_26Q4-release/cf_x86_64_auto-user"
      },
      "instances": [
        {
          "https://github.com/import": "auto",
          "disk": {
            "default_build": "@ab/git_26Q4-release/cf_x86_64_auto-user"
          },
          "boot": {
            "use_vendor_boot_debug": true
          }
        }
      ]
    }

  Then run: `cvd create --config_file config.json`
  After the target boots, you should be able to run `adb root`.