[tasks] Use ParamSpec in ext-tasks

[bijij]

Summary

Uses paramspec to allow loop.call, loop.start and loop.restart to be aware of expected arguments.

Checklist

• [x] If code changes were made then they have been tested.
  • [ ] I have updated the documentation to reflect the changes.
• [ ] This PR fixes an issue.
• [ ] This PR adds something new (e.g. new method or parameters).
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [ ] This PR is not a code change (e.g. documentation, README, ...)