[FEATURE] Return pushPayload result

[esbanarango]

This PR adds a return value (the record(s) that was created or updated) to pushPayload.

This was already done here: https://github.com/emberjs/data/pull/5265/files but apparently was mistakenly removed here https://github.com/emberjs/data/pull/5384/files

[jonnii]

I think this was feature flagged and removed on purpose if my memory serves correctly.

[runspired]

@jonnii is correct. This feature never landed and we decided not to push it forward.

[esbanarango]

@runspired do we have any context on why the decision was made?

[runspired]

@esbanarango many factors

• the implementation had gone stale and didn't have a champion
• the change is complicated semantically as there's no ability to determine whether primary or all records should be returned
• we want to move away from pushPayload entirely
• a public API for this already exists

[esbanarango]

@runspired Thank you 🙏

[runspired]

@esbanarango there is even an open PR atm that will update the docs to actively discourage use of it: #6451 (comment) :)