[WIP] pool StringBuilder with ObjectPool

src/Microsoft.AspNetCore.ResponseCaching/Interfaces/IResponseCachingCacheKeySuffixProvider.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNetCore.Http;
+
+namespace Microsoft.AspNetCore.ResponseCaching
+{
+    public interface IResponseCachingCacheKeySuffixProvider
+    {
+        /// <summary>
+        /// Create a key segment that is appended to the default cache key.
+        /// </summary>
+        /// <param name="httpContext">The <see cref="HttpContext"/>.</param>
+        /// <returns>The key segment that will be appended to the default cache key.</returns>
+        string CreateCustomKeySuffix(HttpContext httpContext);
+    }
+}

src/Microsoft.AspNetCore.ResponseCaching/Interfaces/IResponseCachingCacheabilityValidator.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNetCore.Http;
+
+namespace Microsoft.AspNetCore.ResponseCaching
+{
+    public interface IResponseCachingCacheabilityValidator
+    {
+        /// <summary>
+        /// Override default behavior for determining cacheability of an HTTP request.
+        /// </summary>
+        /// <param name="httpContext">The <see cref="HttpContext"/>.</param>
+        /// <returns>The <see cref="OverrideResult"/>.</returns>
+        OverrideResult RequestIsCacheableOverride(HttpContext httpContext);
+
+        /// <summary>
+        /// Override default behavior for determining cacheability of an HTTP response.
+        /// </summary>
+        /// <param name="httpContext">The <see cref="HttpContext"/>.</param>
+        /// <returns>The <see cref="OverrideResult"/>.</returns>
+        OverrideResult ResponseIsCacheableOverride(HttpContext httpContext);
+    }
+}

src/Microsoft.AspNetCore.ResponseCaching/Internal/NoopCacheKeySuffixProvider.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNetCore.Http;
+
+namespace Microsoft.AspNetCore.ResponseCaching.Internal
+{
+    internal class NoopCacheKeySuffixProvider : IResponseCachingCacheKeySuffixProvider
+    {
+        public string CreateCustomKeySuffix(HttpContext httpContext) => null;
+    }
+}

src/Microsoft.AspNetCore.ResponseCaching/Internal/NoopCacheabilityValidator.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using Microsoft.AspNetCore.Http;
+
+namespace Microsoft.AspNetCore.ResponseCaching.Internal
+{
+    internal class NoopCacheabilityValidator : IResponseCachingCacheabilityValidator
+    {
+        public OverrideResult RequestIsCacheableOverride(HttpContext httpContext) => OverrideResult.UseDefaultLogic;
+
+        public OverrideResult ResponseIsCacheableOverride(HttpContext httpContext) => OverrideResult.UseDefaultLogic;
+    }
+}

src/Microsoft.AspNetCore.ResponseCaching/OverrideResult.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.AspNetCore.ResponseCaching
+{
+    public enum OverrideResult
+    {
+        /// <summary>
+        /// Use the default logic for determining cacheability.
+        /// </summary>
+        UseDefaultLogic,
+
+        /// <summary>
+        /// Ignore default logic and do not cache.
+        /// </summary>
+        DoNotCache,
+
+        /// <summary>
+        /// Ignore default logic and cache.
+        /// </summary>
+        Cache
+    }
+}