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ARROW-13482: [C++][Compute] Refactoring away from hard coded ExecNode factories to a registry

- An extensible registry of exec node factories (`std::function<Result<ExecNode*>(ExecPlan* plan, std::vector<ExecNode*> inputs, const ExecNodeOptions& options)>`) is provided
- Hard coded factories like `compute::MakeSinkNode`, `dataset::MakeScanNode` are replaced by factories in the registry named "sink", "scan", etc
- `arrow::compute::Declaration` is provided to represent an unconstructed set of `ExecNode`s, which can be validated and emplaced into an `ExecPlan` as a unit

Closes #10793 from bkietz/exec-node-factory-registry

Authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>