refactor: move ValidatedInput to the Input namespace

[michalsn]

Description
This PR moves ValidatedInput from CodeIgniter\Validation to CodeIgniter\Input. The class is a typed input data object, not a validator or a validation rule, so it belongs with the other input types. Validation and FormRequest still expose validated data through getValidatedInput(). The difference is that the returned object now sits next to InputData and InputDataFactory instead of among the validation classes.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide