Normalizer: add common snake_case and camelCase keys transformers

[thePanz]

Added common normalizers

• CamelCaseKeysTransformer
• SnakeCaseKeysTransformer
• CustomDateTimeTransformer

[romm]

Hi @thePanz, thanks for the PR!

I'll take some time to think whether or not I want this in the core. If we merge it, we probably have to adapt the documentation as well.

[thePanz]

Thanks 👍
... and be aware of: #764 as I discovered while implementing those 😓

[thePanz]

@romm to add on this: I would add some tests too, and apply the documentation changes before merging.
(I also have a CustomDateTimeTransformer which I would like to add)

Pro of having them in core:

1. code reuse, so others can benefit from those common-usecases
2. cover their usage with tests, avoiding NormalizerBuilder::registerTransformer() with invokable class #764 in the first place (and having examples documented as tests too 😊 )
3. the Symfony bundle could pre-package some of those, or having them configured via the Symfony configuration system (just an idea)

[romm]

👍

The CustomDateTimeTransformer would also make sense if we were to add them in the core.

Just so you known, there is an existing test (that should be moved/adapted): https://github.com/CuyZ/Valinor/blob/194c0cdfa7bc3f23e3c5d254f42acf3ecef5a46a/tests/Integration/Normalizer/CommonExamples/ObjectKeysToSnakeCaseTest.php