[Backport release-1.18] Add support for async workflow activities

[dapr-bot]

Backport of #1053 to release-1.18.

Backporting failed due to merge conflicts. Please resolve manually:

git fetch origin
git checkout backport-1053-to-release-1.18

# Reset to base, discarding the empty placeholder commit
git reset --hard origin/release-1.18

# Cherry-pick the merged commit
git cherry-pick -x -m 1 8367be531680bc5dbbdf1f537b277de899a02ed4

# Resolve the conflicts, then:
git add .
git cherry-pick --continue

# Force-push to replace the placeholder commit
git push --force-with-lease origin backport-1053-to-release-1.18