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ARROW-9815: [Rust][DataFusion] Add a trait for looking up scalar functions by name

Inspired by the conversation on https://github.com/apache/arrow/pull/8018/files, I have been bothered by the use of Arc/Mutex and the resulting code complication in ExecutionContext and LogicalPlanning.

The more I read the code, the more I am convinced that `ExecutionContextState` needs to be mutable *before* planning, but once planning has started the state is all read only (and any relevant state is copied / cloned into the `ExecutionPlan`) so the only thing Mutex/Arc are doing is making lurking bugs like ARROW-9815 more likely and requiring copies.

This PR proposes a modest change to add a trait for looking up scalar functions by name, and thus removes the direct use of Box / HashMaps / etc in the TypeCoercion optimizer pass.

I have several other changes in mind to avoid the need for Box / Mutex entirely in ExecutionContext but I want to keep the individual PRs small.

Closes #8031 from alamb/alamb/execution_context_clean

Authored-by: alamb <andrew@nerdnetworks.org>
Signed-off-by: Andy Grove <andygrove73@gmail.com>