…mentations and handle global optimizations (HeahDude)

This PR was merged into the 5.1-dev branch.

Discussion
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[Form] Added an AbstractChoiceLoader to simplify implementations and handle global optimizations

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | ~
| License       | MIT
| Doc PR        | ~
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Taking over #33218 (taking over #30983)

The `ChoiceLoaderInterface` is not easy to understand/implement, while its goal is simple, lazy load an array of choices.
What may seem complicated is the how/what to optimize loading, we need to deal with a `$value` callback (which refers to the `choice_value` option allowing to transform a model choice to a unique string value that can be displayed/submitted).

We have now enough implementations in core to justify the need of an abstraction and provide a better DX in the process.

Before this PR we needed to implement 3 methods to create a loader:
 - `loadChoiceList(?callable $value): ChoiceListInterface`
 - `loadChoicesForValues(array $values, ?callable $value): array`
 - `loadValuesForChoices(array $choices, ?callable $value): array`
and handle optimization, in each.

Now we only need to implement:
 - `loadChoices():  iterable`

and optionnally:
 - `doLoadChoicesForValues(array $values, ?callable $value): array`
if more optimization is needed to only load submitted values, (i.e the core intl loader prevents loading values that are the same as choices, and the doctrine one performs a `WHERE id IN ($ids)` query).

Commits
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1394df2 [Form] Added an AbstractChoiceLoader to simplify implementations and handle global optimizations