[ping_directory] Add support of User and Group datastream

Collects PingDirectory user identity and group membership records via the SCIM v2 API (CEL input). Covers user accounts (username, email, display name, title, user type) and groups (name, members, metadata).

Includes:

CEL data collection from /scim/v2/Users and /scim/v2/Groups (with /directory/v1/authenticate)
Ingest pipelines with ECS mappings (user.id, user.name, user.email, user.full_name, group.id, group.name, event.category, event.type)
Field definitions, pipeline and system tests, sample events, latest-user transform, and documentation
Dashboards covering user sync over time, users by department and employment type, group counts and memberships, groups over time, and top users.

Part of https://github.com/elastic/integrations/issues/19103.