[WIP] Start prototyping support for opaque engine expressions

[scovich]

What changes are proposed in this pull request?

Kernel does not provide any way for engines to work with expressions kernel does not fully understand. This creates unwelcome pressure to implement operations various engines need, directly in kernel, even tho kernel itself does not use them at all. A canonical example is [NOT] IN, which is highly complex to implement fully and correctly (see e.g. #652), and which was only added in the first place because engines wanted to use it for their own query processing.

The solution is to define an opaque expression trait which engines can use to round-trip any expression they want through the kernel. Initially, kernel can support partition pruning over such expressions if the engine provides an appropriate implementation. Eventually, it might be possible to also express basic data skipping support as well -- but any such effort would be far out of scope for this initial exploration.

This PR affects the following public APIs

Adding more variants to the Expression enum, along with corresponding API support for manipulating it.

How was this change tested?

TODO!

[codecov]

Codecov Report

Attention: Patch coverage is 8.83978% with 165 lines in your changes missing coverage. Please review.

Project coverage is 83.37%. Comparing base (eedfd47) to head (fccd3b7).

Files with missing lines	Patch %	Lines
kernel/src/engine/arrow_expression.rs	0.00%	53 Missing ⚠️
kernel/src/expressions/mod.rs	28.07%	39 Missing and 2 partials ⚠️
kernel/src/predicates/mod.rs	0.00%	33 Missing ⚠️
ffi/src/expressions/kernel.rs	0.00%	17 Missing ⚠️
kernel/src/predicates/parquet_stats_skipping.rs	0.00%	7 Missing ⚠️
kernel/src/scan/data_skipping.rs	0.00%	7 Missing ⚠️
kernel/src/lib.rs	0.00%	5 Missing ⚠️
kernel/src/engine/parquet_row_group_skipping.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #686      +/-   ##
==========================================
- Coverage   84.09%   83.37%   -0.73%     
==========================================
  Files          77       77              
  Lines       17805    17962     +157     
  Branches    17805    17962     +157     
==========================================
+ Hits        14973    14975       +2     
- Misses       2117     2271     +154     
- Partials      715      716       +1     

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