FEAT-#7523: Improve formal definition of the automatic switching algorithm (#7524)

We add the move_to_me_cost function as something to be consulted during
automatic switching. This allows for the /other/ query compiler to have
more of a say in a potential data migration. This also helps to
formalize the questions being asked of each participating query
compiler, specifically the move_to_cost can be precisely defined as just
the transmission and serialization cost of data movement.

We also allow ourselves to disregard transmission cost, or the
move_to_cost when the current engine is simply unable to execute the
current workload.

We also modify the Backend environment variable to allow for setting and
getting the choices in order to constrain the set of engines considered
during automatic switching.

In a future commit we will implement a default function similar to what
is configured in the tests.
A separate future commit will add a public method to set the active
backends.

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## What do these changes do?

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- [x] first commit message and PR title follow format outlined
[here](https://modin.readthedocs.io/en/latest/development/contributing.html#commit-message-formatting)
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- [x] passes `flake8 modin/ asv_bench/benchmarks scripts/doc_checker.py`
- [x] passes `black --check modin/ asv_bench/benchmarks
scripts/doc_checker.py`
- [x] signed commit with `git commit -s` <!-- you can amend your commit
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- [x] Resolves #7523 
- [x] tests added and passing
- [x] module layout described at `docs/development/architecture.rst` is
up-to-date <!-- if you have added, renamed or removed files or
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Co-authored-by: Mahesh Vashishtha <mahesh.vashishtha@snowflake.com>