FEAT-#7459: Add methods to get and set backend.

[sfc-gh-mvashishtha]

Add get_backend() to get the backend for a dataframe or series. Add set_backend(), and its alias move_to(), to set the backend of a dataframe or series.

To implement set_backend(), extend FactoryDispatcher so that it can dispatch I/O operations to the backend that the user chooses instead of always using modin.config.Backend. set_backend() can then use FactoryDispatcher.from_pandas(backend=new_backend) to get a query compiler with the given backend.

This commit also updates the documentation for "native" execution mode to reflect the updated guidance of using Backend to control execution. It also adds examples of using get_backend() and set_backend().

• first commit message and PR title follow format outlined here

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• passes flake8 modin/ asv_bench/benchmarks scripts/doc_checker.py
• passes black --check modin/ asv_bench/benchmarks scripts/doc_checker.py
• signed commit with git commit -s
• Resolves Add DataFrame and Series methods to get and set backend. #7459
• tests added and passing
• module layout described at docs/development/architecture.rst is up-to-date

[sfc-gh-jkew]

I think I'm seeing the same issue