Pass root directory URL as env variable so that plugins can use it

[JaewonHur]

This PR adds a PluginStateRoot class so that a plugin can figure out its root URL where the plugin specific data should be stored.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

[Why is this change needed?]

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

[jglogan]

Using request changes to prevent merge for one sec - why can't the plugin root be determined by convention as ${APP_ROOT}/plugin-state? What is the use case for needing to locate it in some arbitrary location?

[JaewonHur]

I guess the reason I pass it as env variable is that we get pluginStateRoot as an argument in registerWithLaunchd (Sources/ContainerPlugin/PluginLoader.swift:L212).

I was not sure it should be always the default path.

[jglogan]

I think we only need environment variables for our base directories:

• The installation base for the application.
• The base application data directory.
• A base directory for logging, when writing file based logs (this is really a develop/debug capability, not good for production as the logs grow unbounded)