Provide a way to script finding where Spin keeps data

From #2555. The context is bundling as part of a third party installer and still making sure that everything is set up nicely (and in isolation!).

Where does spin store the plugins and templates

Ideally I wouldn't need to know, but since I can't use the cli to query if templates and plugins have been installed or not, I needed to check the filesystem.

I could not find a way to find the directory from the command itself, like I can do with e.g. docker:

$ docker info -f json | jq -r .DockerRootDir
/var/lib/docker

It would be nice to have a similar spin info subcommand that provides the data directory location, e.g.

$ spin info --json | jq -r .SpinAppData
/Users/jan/Library/Application Support/spin

The rules for locating the data directory are complex

Since I can't query the spin app data location, I have to hard-code the rules from the spin app into the installer:

Linux: $HOME/.local/share/spin
macOS: $HOME/Application Support/spin
Windows: %LOCALAPPDATA%\spin

Unfortunately it gets more complicated: On Linux it will be $XDG_DATA_HOME/spin if XDG_DATA_HOME is defined. https://github.com/fermyon/spin/pull/1494 makes the location dependent on installation by Homebrew, and https://github.com/fermyon/spin/issues/641 promises to make things even more complex.

I think replicating all these rules in a 3rd-party installer is bad, but it gets even worse when you consider GUI applications. They are not started from the user's shell, but e.g. on macOS from launchd on behalf of Finder.app. Those apps don't inherit the user's shell environment variables. You could try to look up the default shell for the user from /etc/passwd and then run a login shell as a subprocess to query the environment variables, but this is getting just too convoluted, and is still not guaranteed to be correct 100% of the time either.

Start by tracing how the Spin CLI currently locates its app-data directory and how subcommands expose information. Compare the platform and environment-variable rules described in the issue, then define completion as a queryable info command that reports the effective data directory for the current installation.