"Script" Processor should be able to operate with only cached sessions

[strawgate]

Describe the enhancement:
The script processor has a configuration to pre-create a number of cached sessions. This greatly improves performance under load by making more instances of the processor available. Once all the cached sessions are busy, new Javascript sessions are created, sometimes one per Event.

Describe a specific use case for the enhancement or feature:
It would be nice to have an option to require the use of the cached sessions and to block the pipeline until a cached session is available instead of mass creating new sessions when under load.

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)