Make it possible to nest runtime components (#2909)

Runtime plugins need to be able to wrap components configured in other
runtime components. For example, one runtime plugin should be able to
wrap the HTTP connector configured in another runtime plugin.

This PR makes this possible by:
- Introducing the ability to order runtime plugins within the
service/operation plugin "levels".
- Adding an argument to `RuntimePlugin::runtime_components` so that
implementations can reference components configured by previous plugins.

The `order` function has three separate order values: `Defaults`,
`Overrides`, and `NestedComponents`. The `Defaults` order is currently
unused, but will be used later when we refactor how defaults in config
work. Everything defaults to `Overrides` since most runtime plugins will
want to be in this slot. The `NestedComponents` order is specifically
for runtime plugins that want to create nested components, and runs at
the very end.

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