Refactor physical create_initial_plan to iteratively & concurrently c…

…onstruct plan from the bottom up (#10023)

* Refactor physical create_initial_plan to construct bottom up

* Refactor node mapping into separate function

* Experiment with concurrent bottom up physical planning

* Refactoring and comments

* Remove unnecessary collect

* Preallocate vector capacity

* Remove children.clone()

* Introduce ChildrenContainer enum

* Formatting

* Fix case where extension may have 0 or 1 children

* Documentation and cleanup unwraps

* Minor changes

- Use tokio::Mutex in async environment
- Remove Option from enum, since it is only used for taking.

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Co-authored-by: metesynnada <[email protected]>