Change ExtendedContentManager method accessors to protected

[AristurtleDev]

Description

This PR changes the accessibility of four utility methods in ExtendedContentManager from private to protected, enabling developers to create derived classes without unnecessary code duplication. Additionally, XML documentation has been added to all previously undocumented members.

Related Issues/Tickets

• Closes Change ExtendedContentManager method accessors #973

Changes Made

• Changed GetStream(string path) from private to protected
• Changed CacheAsset(string name, object obj) from private to protected
• Changed NoExtension(string name) from private to protected
• Changed TryGetCachedAsset<T>(string name, out T asset) from private to protected
• Added documentation to all members missing documentation
• Added documentation to the methods that have changed from private to protected
• Fixed issue with split documentation for the LoadTexturePacker between FNA/KNI and MonoGame implementations

Checklist

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