[bug]: Use enums

### Description

Use enums (https://www.php.net/manual/en/language.types.enumerations.php) wherever possible, e.g. in OperationType. The `setOperation` method of `OperationIndexParams `expects a parameter of type `OperationType`, but `MOVE` and `COPY` are currently defined as a string, so calling

```
(new OperationIndexParams())
->setOperation(OperationType::MOVE)
```

generates the warning `Expected parameter of type '\Algolia\AlgoliaSearch\Model\Search\OperationType', 'string' provided`.

### Client

All

### Version

4.15.0

Inspect the OperationType definition and the setOperation method on OperationIndexParams, then reproduce the warning with the example call in the issue. The work is done when MOVE and COPY are represented as OperationType values and the call no longer reports a string type mismatch.

Written by the indexing model from the issue text.