Define and use new AsAny trait

[scovich]

What changes are proposed in this pull request?

As part of implementing various kernel operations, engines need the ability to downcast kernel engine traits to their true concrete types. The rust Any trait can help with struct downcasting, but it isn't quite strong enough once traits are involved. For example, given trait Foo: Any + Send + Sync, and struct Bar that implements Foo, the following fails to compile:

let f: Arc<dyn Foo> = Arc::new(Bar);
let f: Arc<Bar> = f.downcast().unwrap(); // `Arc::downcast` method not found 

... as does this:

let f: Arc<Foo> = Arc::new(Foo{});
let a: Arc<dyn Any + Send + Sync> = f; // trait upcasting coercion is not stable rust
let f: Arc<Foo> = a.downcast().unwrap();

We solve the trait downcasting problem with a new trait AsAny: Any + Send + Sync -- automatically implemented for all T: Any + Send + Sync -- which defines methods for downcasting itself to dyn Any + Send + Sync (reference, box, or arc).

This PR affects the following public APIs

The various engine traits (ParquetHandler, EngineData, etc.) are now constrained to be AsAny (which implies Any + Send + Sync) rather than merely Send + Sync).

This change should be source compatible in the vast majority of cases, because most types implement Any automatically.

How was this change tested?

Doc tests, and it also replaces similar special-case code in the arrow default engine data implementation.

[codecov]

Codecov Report

Attention: Patch coverage is 53.84615% with 6 lines in your changes missing coverage. Please review.

Project coverage is 79.60%. Comparing base (b3ab778) to head (ec6968f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
kernel/src/lib.rs	50.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #450      +/-   ##
==========================================
- Coverage   79.64%   79.60%   -0.04%     
==========================================
  Files          57       57              
  Lines       12324    12330       +6     
  Branches    12324    12330       +6     
==========================================
  Hits         9815     9815              
- Misses       1981     1987       +6     
  Partials      528      528              

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[scovich]

Apparently our codecov doesn't consider doc tests?

Because the doc test just a few lines above definitely exercises the supposedly not-covered method.

[nicklan]

Thanks, this will be useful

[zachschuermann]

regarding coverage of doctests I made an issue for us to follow up: #484

[zachschuermann]

LGTM only question is does AsAny communicate enough information about our traits? Maybe consider calling SendSyncAny or something?

[scovich]

LGTM only question is does AsAny communicate enough information about our traits? Maybe consider calling SendSyncAny or something?

I'm not sure the "fatter" name is really helpful? AsAny requires Send+Sync in order to support Arc, but I suppose we could do AsAny + Send + Sync at all the various sites just for extra clarity?

[zachschuermann]

yea agree - could just leave and make that change later if we think it's necessary :)