[elastic_security] Align Process Fields with ECS Schema

[mohitjha-elastic]

Description:
We need to address inconsistencies between our process.* fields and the Elastic Common Schema (ECS):

Several process-related fields are not currently importing ECS definitions. These should use external: ecs to ensure alignment with the ECS schema and avoid manually duplicating field definitions.

Fields such as process.*.supplemental_groups.id are being ingested as numeric values but ECS expects them as keyword (string). Additionally, supplemental_groups is an array of objects, which differs from ECS expectations and requires explicit nested or group mapping.

Issue discussion here - #14305 (comment)

Proposed Fix:
Apply external: ecs to relevant process fields.

Convert all *.supplemental_groups.id fields to keyword in ingest pipeline.

Ensure supplemental_groups, previous, and similar fields are explicitly defined as nested or group in the field mapping.

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)