Added timestamps, Unix modes, performance improvements and dedicated for Fake File/Directory.

- Add comprehensive timestamp and Unix file mode support across IFile/IDirectory interfaces
- Implement LastWriteTimeUtc, CreationTimeUtc, LastAccessTimeUtc, and UnixFileMode properties
- Add Set* methods for timestamp and Unix file mode manipulation
- Improve performance by replacing GetFiles/GetDirectories with EnumerateFiles/EnumerateDirectories
- Create dedicated Cake.Testing.Tests project with comprehensive unit test coverage
- Add snapshot testing with Verify for FakeFile and FakeDirectory operations
- Fill gaps in file system abstraction with cross-platform Unix file mode support
- Integrate Microsoft.Extensions.TimeProvider.Testing for reliable time handling in tests
- Refactor FakeDirectory to use yield return for lazy evaluation performance benefits
- Add extensive test coverage for Create, Move, Delete, GetDirectories, and GetFiles operations
- fixes #4636

### Breaking Changes
- [x] There shouldn't be any breaking changes identified, all new interface members have default implementations / values.

#### IFileSystemInfo Interface (Base interface)
- [x] `DateTime? LastWriteTimeUtc` - Default: `null`
- [x] `DateTime? CreationTimeUtc` - Default: `null`
- [x] `DateTime? LastAccessTimeUtc` - Default: `null`
- [x] `UnixFileMode? UnixFileMode` - Default: `null`

#### IDirectory Interface
- [x] `IDirectory SetCreationTime(DateTime creationTime)` - Default: `return this`
- [x] `IDirectory SetCreationTimeUtc(DateTime creationTimeUtc)` - Default: `return this`
- [x] `IDirectory SetLastAccessTime(DateTime lastAccessTime)` - Default: `return this`
- [x] `IDirectory SetLastAccessTimeUtc(DateTime lastAccessTimeUtc)` - Default: `return this`
- [x] `IDirectory SetLastWriteTime(DateTime lastWriteTime)` - Default: `return this`
- [x] `IDirectory SetLastWriteTimeUtc(DateTime lastWriteTimeUtc)` - Default: `return this`
- [x] `IDirectory SetUnixFileMode(UnixFileMode unixFileMode)` - Default: `return this`

#### IFile Interface
- [x] `IFile SetUnixFileMode(UnixFileMode unixFileMode)` - Default: `return this`

### Key Changes Made
- [x] Added UTC time properties to IFileSystemInfo with default null values
- [x] Added Unix file mode support with UnixFileMode property and setter methods
- [x] Added time setter methods for both IDirectory and IFile interfaces
- [x] Performance improvement: Changed from GetDirectories()/GetFiles() to EnumerateDirectories()/EnumerateFiles()
- [x] Added _file.Refresh() calls after time-setting operations to ensure consistency
- [x] Added platform-specific attributes ([UnsupportedOSPlatform("windows")]) for Unix-specific methods