fix: use LA pool for the local activities execution

[rustatian]

Reason for This PR

closes: roadrunner-server/roadrunner#1940

Description of Changes

• WF pool was incorrectly used to execute Local Activities.
• Update implementation, use the general pool for A + LA.
• Use LA execute function directly, w/o constructor like in prev. implementation.
• Add headers to the execute function (passed to the worker).

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.