[C++] rename ARROW_ENGINE to ARROW_SUBSTRAIT

[asfimport]

When we introduced substrait we reused the cmake + feature ARROW_ENGINE to mean compute+a few other things as well as the substrait consumer functionality. In general, right now, we don't yet need (or want) to build substrait in our packages (e.g. the R package) since many places don't yet take advantage of it. But the naming of the cmake or feature is now confusing: it effectively is only substrait if you separately enable compute, etc. but it makes it sound like the query engine we have been building since 6.0.0 is disabled.

We should rename ARROW_ENGINE to ARROW_SUBSTRAIT now and then we can add an ARROW_ENGINE later if we need to encompass a larger set of engine functionality (e.g. compute+spillover+scheduler+memory limits) if that's needed.

Reporter: Jonathan Keane / @jonkeane
Assignee: Weston Pace / @westonpace

PRs and other links:

• GitHub Pull Request #12915

_{Note: This issue was originally created as ARROW-16158. Please see the migration documentation for further details.}

[asfimport]

Jonathan Keane / @jonkeane:
I've made this a blocker for 8.0.0 not because the name is super super important, but rather because we have not yet released with this new meaning of ARROW_ENGINE so if we do this before the release there's no change anyone is depending on it (in a released pacakge)

[asfimport]

Antoine Pitrou / @pitrou:
I'm a bit surprised about this request, because ARROW_ENGINE originally meant the execution engine. Perhaps we want a new separate ARROW_SUBSTRAIT build flag?

cc @westonpace  

[asfimport]

Weston Pace / @westonpace:
Yes, we will eventually want both ARROW_ENGINE and ARROW_SUBSTRAIT it sounds like. We had originally named it ARROW_ENGINE thinking that more components would move into here over time. However, those components haven't moved yet. So currently, once you take away the Substrait components there is not much left. I think the idea is to rename to ARROW_SUBSTRAIT and then later introduce ARROW_ENGINE once we have more engine components.

[asfimport]

Antoine Pitrou / @pitrou:
I agree with renaming to ARROW_SUBSTRAIT as it's a more explicit and informative name for the component anyway.

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 12915
#12915