Add apollo federation connector

src/HotChocolate/Fusion/HotChocolate.Fusion.slnx

@@ -13,6 +13,7 @@
     <Project Path="src/Fusion.Language/HotChocolate.Fusion.Language.csproj" />
     <Project Path="src/Fusion.Packaging/HotChocolate.Fusion.Packaging.csproj" />
     <Project Path="src/Fusion.SourceSchema.Packaging/HotChocolate.Fusion.SourceSchema.Packaging.csproj" />
+    <Project Path="src/Fusion.Connectors.ApolloFederation/HotChocolate.Fusion.Connectors.ApolloFederation.csproj" />
     <Project Path="src/Fusion.Connectors.InMemory/HotChocolate.Fusion.Connectors.InMemory.csproj" />
     <Project Path="src/Fusion.Utilities/HotChocolate.Fusion.Utilities.csproj" />
   </Folder>
@@ -26,6 +27,7 @@
     <Project Path="test/Fusion.Language.Tests/HotChocolate.Fusion.Language.Tests.csproj" />
     <Project Path="test/Fusion.Packaging.Tests/HotChocolate.Fusion.Packaging.Tests.csproj" />
     <Project Path="test/Fusion.SourceSchema.Packaging.Tests/HotChocolate.Fusion.SourceSchema.Packaging.Tests.csproj" />
+    <Project Path="test/Fusion.Connectors.ApolloFederation.Tests/HotChocolate.Fusion.Connectors.ApolloFederation.Tests.csproj" />
     <Project Path="test/Fusion.Connectors.InMemory.Tests/HotChocolate.Fusion.Connectors.InMemory.Tests.csproj" />
     <Project Path="test/Fusion.Utilities.Tests/HotChocolate.Fusion.Utilities.Tests.csproj" />
     <Project Path="test/Fusion.Tests.Shared/HotChocolate.Fusion.Tests.Shared.csproj" />

src/HotChocolate/Fusion/src/Fusion.Composition.ApolloFederation/AnalysisResult.cs

@@ -0,0 +1,42 @@
+using HotChocolate.Fusion.Errors;
+using HotChocolate.Language;
+
+namespace HotChocolate.Fusion.ApolloFederation;
+
+/// <summary>
+/// Contains the metadata extracted from analyzing a Federation v2 schema document.
+/// </summary>
+internal sealed class AnalysisResult
+{
+    /// <summary>
+    /// Gets the detected federation version string (e.g. "v2.0", "v2.5").
+    /// A value of "v1" indicates unsupported federation v1.
+    /// </summary>
+    public string FederationVersion { get; set; } = "v1";
+
+    /// <summary>
+    /// Gets the entity key definitions keyed by type name.
+    /// Each type may have multiple <c>@key</c> directives.
+    /// </summary>
+    public Dictionary<string, List<EntityKeyInfo>> EntityKeys { get; init; } = [];
+
+    /// <summary>
+    /// Gets the field type map: typeName -> fieldName -> field type node.
+    /// </summary>
+    public Dictionary<string, Dictionary<string, ITypeNode>> TypeFieldTypes { get; init; } = [];
+
+    /// <summary>
+    /// Gets the name of the query root type (defaults to "Query").
+    /// </summary>
+    public string QueryTypeName { get; set; } = "Query";
+
+    /// <summary>
+    /// Gets the list of composition errors detected during analysis.
+    /// </summary>
+    public List<CompositionError> Errors { get; init; } = [];
+
+    /// <summary>
+    /// Gets a value indicating whether analysis produced any errors.
+    /// </summary>
+    public bool HasErrors => Errors.Count > 0;
+}

src/HotChocolate/Fusion/src/Fusion.Composition.ApolloFederation/EntityKeyInfo.cs

@@ -0,0 +1,18 @@
+namespace HotChocolate.Fusion.ApolloFederation;
+
+/// <summary>
+/// Represents a single <c>@key</c> directive on a federation entity type.
+/// </summary>
+internal sealed class EntityKeyInfo
+{
+    /// <summary>
+    /// Gets the raw field selection string from <c>@key(fields: "...")</c>.
+    /// </summary>
+    public required string Fields { get; init; }
+
+    /// <summary>
+    /// Gets a value indicating whether the key is resolvable.
+    /// Defaults to <c>true</c> when the <c>resolvable</c> argument is omitted.
+    /// </summary>
+    public bool Resolvable { get; init; } = true;
+}

src/HotChocolate/Fusion/src/Fusion.Composition.ApolloFederation/FederationDirectiveNames.cs

@@ -0,0 +1,19 @@
+namespace HotChocolate.Fusion.ApolloFederation;
+
+internal static class FederationDirectiveNames
+{
+    public const string Key = "key";
+    public const string Requires = "requires";
+    public const string Provides = "provides";
+    public const string External = "external";
+    public const string Link = "link";
+    public const string Shareable = "shareable";
+    public const string Inaccessible = "inaccessible";
+    public const string Override = "override";
+    public const string Tag = "tag";
+    public const string InterfaceObject = "interfaceObject";
+    public const string ComposeDirective = "composeDirective";
+    public const string Authenticated = "authenticated";
+    public const string RequiresScopes = "requiresScopes";
+    public const string Policy = "policy";
+}

src/HotChocolate/Fusion/src/Fusion.Composition.ApolloFederation/FederationFieldNames.cs

@@ -0,0 +1,7 @@
+namespace HotChocolate.Fusion.ApolloFederation;
+
+internal static class FederationFieldNames
+{
+    public const string Entities = "_entities";
+    public const string Service = "_service";
+}