[Feature] wire the datalayer plugins into the configuration

[ronenkat]

What type of PR is this?
/kind feature

What this PR does / why we need it:
Replaces the hardcoded NotificationSource in cmd/runner/runner.go with a
config-driven approach. Users now declare notification-source plugins in the plugins
section and list them under a new top-level notificationSources field in the
PayloadProcessorConfig. The runner starts each configured source via the
controller-runtime manager, which guarantees Stop() is called on graceful shutdown.

Changes:

• apix/config/v1alpha1: added NotificationSources []PluginRef field to
  PayloadProcessorConfig (and updated generated DeepCopy)
• pkg/config: added NotificationSources []datasource.NotificationSource to Config
• pkg/config/loader: added buildDatalayer called after buildProfiles; resolves
  plugin refs and type-asserts each to NotificationSource
• internal/runnable: new DataSourceRunnable adapter that starts a DataSource,
  blocks until context cancellation, then calls Stop()
• cmd/runner: loadConfiguration now returns *config.Config; replaces hardcoded
  block with a loop that registers each source with the manager
• deploy/config/ipp-datalayer-config.yaml: example config wiring a notification-source
  with a request-metadata extractor

Which issue(s) this PR fixes:
Fixes #135

Release note (write NONE if no user-facing change):

NotificationSource plugins can now be declared in the IPP configuration file and are
started/stopped automatically as part of the manager lifecycle.

[github-actions]

⚠️ Large PR detected

Your PR is large. Please consider breaking it into multiple PRs.

The do-not-merge/hold label has been added and can be removed by the reviewers based on their judgement.

[shmuelk]

/lgtm
/approve
/hol cancel