## Type of Change
- [x] New feature
- [x] Breaking change
## Motivation and Context
This PR changes the behavior of the `defaultArguments` field in the
service plugin config. Previously, `defaultArguments` was functioning as
a way to indicate how to start a plugin in the event that the plugin is
loaded at boot (such as for the `container-core-images plugin`).
However, we now follow a convention where all plugins have a "start"
command that is used when launching the plugin, so this
`defaultArguments` field wasn't really providing much. Instead, there
are use cases where we may want to set default values to pass to a
plugin. This PR repurposes the `defaultArguments` field for those use
cases.
As an example use case, there are scenarios where someone may want to
use the AllocationOnlyVmnetNetwork even when running on macOS 26+. This
PR adds the ability to pass in a command line option to the vmnet
network plugin to specify that request. Combined with the
`defaultArguments` plugin config change, a user may choose to set that
field to ["--variant", "allocationOnly"] in the
`container-network-vmnet-config.json` to use AllocationOnlyVmnetNetwork
by default for all networks.
## Testing
- [x] Tested locally
- This helps with brew cask packaging, can postflight link
libexec/container/plugins to the appropriate cask location, and the
plugin discovery will traverse the link.
- Sets up API server as source of truth for installation root, similarly
to what was done for the data root. `system start` establishes the
install root, setting the environment variable `CONTAINER_INSTALL_ROOT`
when launching the API server.
- The API server propagates the environment variable when launching
helpers, and returns the install root to the CLI via the health check
XPC.
- Includes several fixes for detecting plugins that use app bundle
layout.
Kathryn Baldauf