[Foundation] Fix nullability in NSStream.

This is file 18 of 47 files with nullability disabled in Foundation.

Changes:
- Enabled nullable reference types
- Removed four [SupportedOSPlatform] attributes without version numbers from NSStreamSocksOptions class
- Made fields in NSStreamSocksOptions nullable where appropriate (HostName, Username, Password)
- Made return types nullable: indexer, SocksOptions, DataWrittenToMemoryStream, FileCurrentOffset, and out parameters in CreatePair methods
- Replaced xml include directive with inline documentation from Item(Foundation.xml file
- Removed Item(Foundation.xml file as it only contained the now-inlined documentation
- Replaced "To be added." XML documentation with proper documentation for all methods and properties
- Added see cref attributes throughout for better cross-referencing
- Removed unnecessary whitespace in XML comments
- Replaced manual null checks with ArgumentNullException.ThrowIfNull
- Fixed typo in example code (Passowrd -> Password)
- Improved formatting and consistency in XML documentation

Contributes towards #17285.

Author: GitHub Copilot