windows.powershell.operational log source properly populates winlog.process.pid that pid IS the powershell process pid, so promote it to the ecs process.pid field like how we do for sysmon and windows defender.

https://github.com/elastic/integrations/blob/451750a1a9e8ad115f99013c83bebfd04124ec12/packages/windows/data_stream/sysmon_operational/elasticsearch/ingest_pipeline/default.yml#L345-L351
https://github.com/elastic/integrations/blob/a74f0bf780d1d4a39986bca9bf2f1de1ba04e4ec/packages/windows/data_stream/windows_defender/elasticsearch/ingest_pipeline/default.yml#L160-L164