Add NuGet.config for centralized package management

NuGet.config

@@ -0,0 +1,35 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+    <packageSources>
+        <add key="dotnet7" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet7/nuget/v3/index.json" />
+        <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
+        <add key="npgsql-vnext" value="https://www.myget.org/F/npgsql-vnext/api/v3/index.json" />
+        <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+    </packageSources>
+
+    <packageSourceMapping>
+        <packageSource key="nuget.org">
+            <package pattern="*" />
+            <!-- Our npgsql-vnext feed doesn't necessarily contain the last preview (package retention rules...), so we take it from nuget.org -->
+            <package pattern="Npgsql" />
+            <package pattern="Npgsql.*" />
+        </packageSource>
+        <packageSource key="npgsql-vnext">
+            <package pattern="Npgsql" />
+            <package pattern="Npgsql.*" />
+        </packageSource>
+        <packageSource key="dotnet7">
+            <package pattern="*" />
+        </packageSource>
+        <packageSource key="dotnet-eng">
+            <!-- EF Core depends on xunit 2.4.2-pre.9 which is only in dotnet-eng.
+                 But some xunit packages still need to come from nuget.org, so we cherry-pick. -->
+            <package pattern="xunit" />
+            <package pattern="xunit.assert" />
+            <package pattern="xunit.core" />
+            <package pattern="xunit.extensibility.core" />
+            <package pattern="xunit.extensibility.execution" />
+        </packageSource>
+    </packageSourceMapping>
+
+</configuration>