Fix ScheduleMessage not returning scheduled_message_id

I'd like to introduce the fix for ScheduledMessage API. Currently, it doesn't return scheduled_message_id where this ID is needed for cancelling the scheduled message. My change is the least intrusive I could think of. It doesn't introduce any new field or adding new API. Basically, this change should be backward compatible. It is based on the fact that the API chat.postMessage doesn't return scheduled_message_id and the API chat.scheduledMessage doesn't return either message.ts or ts. We just need to check the presence of this field and determine the caller API based on that.

Existing code shouldn't break because the timestamp is always an empty string for ScheduledMessage API. Now it will return scheduled_message_id in place of timestamp. SendMessage should remain the same.

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Should this be an issue instead

[ ] is it a convenience method? (no new functionality, streamlines some use case)
[ ] exposes a previously private type, const, method, etc.
[ ] is it application specific (caching, retry logic, rate limiting, etc)
[ ] is it performance related.

API changes

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no tests, if you're adding to the API include at least a single test of the happy case.
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dependency changes. updates are okay. adding/removing need justification.

Examples of API changes that do not meet guidelines:

in library cache for users. caches are use case specific.
Convenience methods for Sending Messages, update, post, ephemeral, etc. consider opening an issue instead.

slack-go / slack