Add support for opaque expressions

[scovich]

Please describe why this is necessary.

Kernel does not currently provide any way for engines to use expressions kernel does not fully understand. This creates unwelcome pressure to implement -- directly in kernel -- operations that kernel does not use but which various engines need. A canonical example is [NOT] IN, which is highly complex to implement fully and correctly (see e.g. #652). Kernel does not actually use it, but it was added because engines wanted to use it for their own query processing.

Describe the functionality you are proposing.

Define an opaque expression trait which engines can use to round-trip any expression they want through the kernel. Initially, kernel could support partition pruning over such expressions if the engine provides an appropriate scalar evaluation implementation. Eventually, it might be possible to also express basic data skipping support as well.

Additional context

No response