Add code infrastructure to validate file content (#14657)

* Implemented modular architecture for filestream security sanitization with an svg-html example

* 31440: Refactoring, applied to more entry points and removed test analyzer

* 31440 Added Unittests for FileStreamSecurityValidator

* PR fixes and better unittest mock names

---------

Co-authored-by: Sven Geusens <sge@umbraco.dk>

src/Umbraco.Core/Security/IFileStreamSecurityValidator.cs

@@ -0,0 +1,11 @@
+namespace Umbraco.Cms.Core.Security;
+
+public interface IFileStreamSecurityValidator
+{
+    /// <summary>
+    /// Analyzes wether the file content is considered safe with registered IFileStreamSecurityAnalyzers
+    /// </summary>
+    /// <param name="fileStream">Needs to be a Read seekable stream</param>
+    /// <returns>Whether the file is considered safe after running the necessary analyzers</returns>
+    bool IsConsideredSafe(Stream fileStream);
+}