## Description
This PR introduces a new `Diff<T>` class that computes the set difference operation (A - B) between two sets. The class extends `SetEnvelope<T>` and provides multiple constructors to support different input types:

- Iterable<T>
- Iterator<T>
- Scalar<Iterable<T>>
- SetOf<T>

The implementation follows the Cactoos object-oriented approach and includes comprehensive test coverage.

## Implementation Details
- The `Diff` class computes elements that are in the first set but not in the second set
- Added multiple constructors to support various input types
- Implemented a private `computeDiff` method to handle the actual set difference operation
- Added thorough test cases covering various scenarios:
  - Basic set difference
  - Empty set handling (both first and second sets)
  - Working with java.util.Set
  - Working with Iterables and Iterators

## Test Coverage
The implementation includes tests for all edge cases and typical usage scenarios:
- Computing difference between two non-empty sets
- Computing difference with an empty second set
- Computing difference with an empty first set
- Working with java.util.Set instances
- Working with Iterables and Iterators

All tests pass successfully, demonstrating the correctness of the implementation.