ARROW-9977: [Rust] Added min/max of [Large]StringArray

[jorgecarleitao]

Like the title says, another helper.

Note that, contrarily to horizontal operations, that we can represent as f(&ArrayRef) -> ArrayRef, in vertical operations like min, and sum we do not have a dynamically-typed struct to return, and thus need to have a function signature for each type. :(

I wonder if we could have in Rust a dynamically struct for single values, like DataFusion has ScalarValue.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-9977

[nevi-me]

There's a lot of value in creating StructValue in arrow, like cpp has. One of the potential benefits would be showing the compute kennels to either take an array or scalar

I've tried doing this previously, but threw away the work as I couldn't get it quite right.

[jorgecarleitao]

@nevi-me , to understand, the goal would we a scalar that we could safely convert Vec<Scalar> from and to an array? What is the issue with

enum ScalarValue {
    Int32(Option<i32>)
    ...
}

[andygrove]

I think that @nevi-me 's suggestion addresses something that I've been trying to solve as well. Our kernels operate on arrays, so calling a + b is fine when both are arrays, but if we're evaluating a + 1 against a batch of 1000 rows then we have to create an array of 1000 literal ones to pass to the kernel.

[andygrove]

LGTM