Expose ingest metadata field `timestamp` in script processor

[stu-elastic]

The script processor has access to metadata fields _type, _id, _version_type, _version and _index.

timestamp metadata value is available to other processors but not the script processor.

Can we expose this metadata field in the script processor as well?

[elasticmachine]

Pinging @elastic/es-data-management (Team:Data Management)

[joegallo]

This isn't a duplicate of #60470, but it's near enough to that one that anybody who looks at this ticket should probably know about that one.