[chore][extension] update NotifyConfig contract (#9005)

**Description:**
The collector implementation makes a copy of effective configuration
before calling `NotifyConfig` so it's safe for the config watcher to
save the pointer and use it later. It is resonable to promise this in
the API contract.

**Link to tracking Issue:**
open-telemetry/opentelemetry-collector-contrib#29277 (comment)

---------

Co-authored-by: Evan Bradley <11745660+evan-bradley@users.noreply.github.com>

extension/extension.go

@@ -45,6 +45,9 @@ type PipelineWatcher interface {
 // wishes to be notified of the Collector's effective configuration.
 type ConfigWatcher interface {
 	// NotifyConfig notifies the extension of the Collector's current effective configuration.
+	// The extension owns the `confmap.Conf`. Callers must ensure that it's safe for
+	// extensions to store the `conf` pointer and use it concurrently with any other
+	// instances of `conf`.
 	NotifyConfig(ctx context.Context, conf *confmap.Conf) error
 }