[Processor\Script] Add option to prevent creation of new sessions

[strawgate]

Fixes #40690

Proposed commit message

Include an option called only_cached_sessions that causes the script processor to only use the pre-defined cached javascript sessions from the session pool. This option is off by default. Includes some unrelated fixes for lint errors related to unchecked errors returned by called functions.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

No

Use cases

Prevent the Beat from temporarily allocating and deleting hundreds of javascript sessions during normal operation.

Allows using more expensive Javascript scripts knowing they will only undergo initialization a single time.

Allows funneling all Beat events through a single session

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @strawgate? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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