Refactor out xcom constants from models

[vemikhaylov]

Sometimes it may be needed to use the constants to have a single source of truth, but we may not want to introduce a dependency on the models layer, where it's possible to avoid that (e.g., in CLI parsing - #29608 (comment)).

Note: Creating it as a separate PR to keep the above mentioned one more focused on the specific --xcoms CLI arg task and facilitate the review.

[vemikhaylov]

When I tried to migrate the providers' code to import airflow.utils.xcom, the Airflow 2.3 compatibility test for them failed (https://github.com/apache/airflow/actions/runs/4456673101/jobs/7827374562?pr=30180) since naturally there was no corresponding package.

I'm not sure what the right way to migrate them is, would appreciate some guidance!

[potiuk]

Figure out a way.

For exampele import the new constants in the old place and mark it with comments to remove it when min-airflow-version is 2.6.

The test is right (and was specifically designed to inform and prevent you from ignoribng the fact that new providers can be used with old airflow (and old providers with new airflow).

[vemikhaylov]

The test is right

Yeah, sure, I didn't have any doubts around that :)

For exampele import the new constants in the old place and mark it with comments to remove it when min-airflow-version is 2.6.

Makes sense to me, I think this is exactly what I came up with, I'll extend the comment with the minimal airflow version according to your proposal then, thanks!

[potiuk]

That looks good now :)