Add labels on components

[markfisher]

The labels are optional key/value pairs to add metadata, e.g.

[component.inventory]
uri = "./inventory.wasm"
labels.domain = "retail"

or:

[component.inventory.labels]
domain = "retail"

These will support at least a couple new features:

1. the revised "scope" attribute with a label-selector (for limiting the set of components where a component or capability having the scope property may be imported)
2. the new [interceptor] component that will select target components for which the interceptor will be applied