add_date processor

[busma13]

This PR makes the following changes:

• add a new processor that will set field to the current date. Use overwrite: true to change the value if field already exists. The date format defaults to iso_8601 and accepts epoch_millis, millisecond, and date-fns format strings.
• bump asset to v2.8.0

[github-actions]

Asset artifacts:
teraslice-test-asset-24-dev.2ded53e: https://github.com/terascope/standard-assets/actions/runs/34644720512/artifacts/10282056035
teraslice-test-asset-22-dev.2ded53e: https://github.com/terascope/standard-assets/actions/runs/34644720512/artifacts/10281316779

[github-actions]

Asset artifacts:
teraslice-test-asset-24-dev.74d2fa9: https://github.com/terascope/standard-assets/actions/runs/35255629391/artifacts/10512500331
teraslice-test-asset-22-dev.74d2fa9: https://github.com/terascope/standard-assets/actions/runs/35255629391/artifacts/10512114965

[github-actions]

Asset artifacts:
teraslice-test-asset-22-dev.7159a48: https://github.com/terascope/standard-assets/actions/runs/35665182316/artifacts/10668892279
teraslice-test-asset-24-dev.7159a48: https://github.com/terascope/standard-assets/actions/runs/35665182316/artifacts/10668572485