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

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`.

Closes #8171 from jorgecarleitao/max_string

Authored-by: Jorge C. Leitao <jorgecarleitao@gmail.com>
Signed-off-by: Andy Grove <andygrove@nvidia.com>