Strongly type timestamps

[rarkins]

Describe the proposed change(s).

Similar to what was done for LongCommitSha, we should add a strongly typed Timestamp type, which is a validated Timestamp. We should also consistently use luxon DateTime capabilities throughout if we're not already.

export type Timestamp = string & { __timestamp: never };

Our datasource logic should be updated to only return releaseTimestamp if it's of type Timestamp. Individual datasource implementations could possibly return invalid strings, but they should be filtered before being returned to the worker layer.