Remove plugin discovery overrides #3803
Description
Overrides are not concurrency safe because they are global and do not use locks (we don't really want to either). There are currently about 25 calls to override definitions.
If we use ResolvablePluginDiscovery: TypeAlias = PluginDiscovery[_PluginDefinitionT] | ResolvableDefinition[_PluginDefinition] | Callable[...], discoveries can not only be chained, but any overrides can be anything from a single definition to an iterable of discoveries.
Instead, allow definitions to be injected into service levels. Can ServiceLevel accept a plugin_manager arg so we have one single way of overriding any plugin type's definitions?