Add event_ts field on all "inner events"

[abustany]

Description

Currently (as of 93035c9) the EventTimestamp field is only present on some inner events types. According to the OpenAPI specs, the event_ts field should be present along with type on all inner events.

(Optional) Slack's documentation

https://github.com/slackapi/slack-api-specs/blob/master/events-api/slack_events_api_async_v1.json#L30

The web docs don't mention that event_ts should be present, but I can verify that it actually is.

[kanata2]

The slack-api-specs repository is not maintained precisely as far as I can see.
So if you want to know the latest specifications, you can look at the official SDKs for Node.js and Java.

[abustany]

ah good thanks for the pointer to the Java SDK. I updated #1052 to not add event_ts on the grid migration events, since those don't seem to have that field according to the Java SDK.

I wonder if one could hack a quick&dirty parser to generate the Go models from the Java ones 🤷