Event definitions: merge identifiers and additional properties

What does this MR do and why?

Implements dynamic extraction of additional properties from method parameters in InternalEvents.track_event

Problem: Currently, developers must pass additional properties when tracking internal events which doesn't bring any value, its just internal tracking logic exposure.

Solution: Introduces a mechanism that automatically extracts properties from kwargs based on the event definition's additional_properties definition

Validation

1. Enable the feature flag:

Feature.enable(:merge_additional_properties_for_snowplow)

2. Test the extraction behavior:

Before - manual specification required

  Gitlab::InternalEvents.track_event(
    'user_created_project',
    additional_properties: { label: 'enterprise', property: 'saas' },
    user: user,
    project: project
  )

After - automatic extraction

  Gitlab::InternalEvents.track_event(
    'user_created_project',
    user: user,
    project: project,
    label: 'enterprise',     # Automatically extracted
    property: 'saas'         # Automatically extracted
  )

3. Run the specs: bundle exec rspec spec/lib/gitlab/internal_events_spec.rb -e "dynamic additional_properties extraction"

Related to #544730 (closed)