Add organization actor support to features API

What does this MR do and why?

Adds support for using organizations as actors when enabling/disabling feature flags via the features API. This follows the same pattern as existing actors (group, project, user, namespace, runner, endpoint, repository).

Organizations are specified by ID, supporting comma-separated multiple IDs.

How to set up and validate locally

# Enable a feature flag for an organization
curl --request POST --header "PRIVATE-TOKEN: <token>" \\
  --data "value=true&organization=1" \\
  "https://gitlab.example.com/api/v4/features/my_feature"

# Enable for multiple organizations
curl --request POST --header "PRIVATE-TOKEN: <token>" \\
  --data "value=true&organization=1,2,3" \\
  "https://gitlab.example.com/api/v4/features/my_feature"

MR acceptance checklist

• Tests added for new functionality
• OpenAPI documentation updated

#498238 (closed)