<PackageReference Include="System.ClientModel" Version="1.12.0" />

System.ClientModel.Primitives.CredentialSectionOverlay

Builds a writable IConfigurationSection wrapper backed by an in-memory dictionary that is initialized from the original credential section. Writes go to the dictionary; reads return the merged view. Setting a key to null removes it (standard IConfiguration semantics).
namespace System.ClientModel.Primitives { internal static class CredentialSectionOverlay { public static IConfigurationSection CreateOverlay(IConfigurationSection original); } }