[C++] Allow Substrait's default extension provider to be configured

[westonpace]

Describe the enhancement requested

A custom extension provider can be specified when running a query. However, in some cases (e.g. when Acero is embedded in a larger program that provides its own extension handlers) it would be simpler to change the default instead of having to specify the custom provider with every query run.

Component(s)

C++

[rtpsw]

take

[rtpsw]

@westonpace, @icexelloss, this issue may be a no-op, because ExtensionProvider::kDefaultExtensionProvider is public and can already be directly set. We could have some kind of guarding around its setting, if desired.

[icexelloss]

IMO kDefaultExtensionProvider is not a public API so we should not change it directly, even if it works today there is no guarantee it will always work after some refactoring. The API can be as simple as changing that variable under the hood but we still need a nice public API for it.

[westonpace]

I agree with @icexelloss . We should move kDefaultExtensionProvider into the .cc file and/or make it const.

[rtpsw]

We're on the same page on that. My question is whether we need anything more, like some kind of guard, in the implementation of the API for setting.

[westonpace]

You mean like a mutex / lock_guard? Yes, that would be a good idea. Or am I misunderstanding?

[rtpsw]

Yes, and wanted to make sure there's nothing else needed that I might be missing.

[westonpace]

I think that would be sufficient.