Allow scaffolders to choose packages/versions by TFM

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1010773/

Author: nosami

main/src/addins/MonoDevelop.AspNetCore/MonoDevelop.AspNetCore.Scaffolding/Configuration/PackageDescription.cs

@@ -0,0 +1,11 @@
+namespace Microsoft.WebTools.Scaffolding.Core.Config
+{
+	class PackageDescription
+	{
+		public string PackageId { get; set; }
+		public string MinVersion { get; set; }
+		public string MaxVersion { get; set; }
+		public bool IsOptionalEfPackage { get; set; } = false;
+		public bool IsOptionalIdentityPackage { get; set; } = false;
+	}
+}

main/src/addins/MonoDevelop.AspNetCore/MonoDevelop.AspNetCore.Scaffolding/Configuration/ScaffoldingConfig.cs

@@ -0,0 +1,94 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Net;
+using System.Net.Http;
+using System.Threading.Tasks;
+using Newtonsoft.Json;
+
+namespace Microsoft.WebTools.Scaffolding.Core.Config
+{
+	class ScaffoldingConfig
+	{
+		public static string ConfigPath { get; private set; } = Path.GetDirectoryName (typeof (ScaffoldingConfig).Assembly.Location);
+
+		public string Version { get; set; }
+
+		// LTS10, FTS11, NetStandard20, NetStandard21, and Net22 packages are set up as they are to maintain backwards compat.
+		// They were (are) explicitly named sections before the config file format was generalized to support arbitrary support policy versions.
+		public PackageDescription [] LTS10Packages { get; set; }
+
+		public PackageDescription [] FTS11Packages { get; set; }
+
+		public PackageDescription [] NetStandard20Packages { get; set; }
+
+		public PackageDescription [] NetStandard21Packages { get; set; }
+
+		public PackageDescription [] Net22Packages { get; set; }
+
+		// This is public so the Json deserialization works (and for testing).
+		// The data should be accessed via TryGetPackagesForSupportPolicyVersion
+		[JsonProperty]
+		public Dictionary<string, PackageDescription []> DynamicVersionedPackages { get; set; }
+
+		public bool TryGetPackagesForSupportPolicyVersion (SupportPolicyVersion supportPolicyVersion, out PackageDescription [] packageDescriptions)
+		{
+			if (supportPolicyVersion == null || supportPolicyVersion.Version == null) {
+				packageDescriptions = null;
+				return false;
+			}
+
+			if (supportPolicyVersion == SupportPolicyVersion.LTS10) {
+				packageDescriptions = LTS10Packages;
+				return true;
+			}
+			if (supportPolicyVersion == SupportPolicyVersion.FTS11) {
+				packageDescriptions = FTS11Packages;
+				return true;
+			}
+			if (supportPolicyVersion == SupportPolicyVersion.NetStandard20) {
+				packageDescriptions = NetStandard20Packages;
+				return true;
+			}
+			if (supportPolicyVersion == SupportPolicyVersion.NetStandard21) {
+				packageDescriptions = NetStandard21Packages;
+				return true;
+			}
+			if (supportPolicyVersion == SupportPolicyVersion.Net220) {
+				packageDescriptions = Net22Packages;
+				return true;
+			}
+
+			if (DynamicVersionedPackages != null && DynamicVersionedPackages.TryGetValue (supportPolicyVersion.Version.ToString (), out packageDescriptions)) {
+				return true;
+			}
+
+			packageDescriptions = null;
+			return false;
+		}
+
+		static ScaffoldingConfig fetchedConfig;
+		// This url will go live for 16.4
+		static string packageVersionsUrl = "https://webpifeed.blob.core.windows.net/webpifeed/partners/scaffoldingpackageversions_2108718.json";
+
+		public static async Task<ScaffoldingConfig> LoadFromJsonAsync ()
+		{
+			if(fetchedConfig == null) {
+				Stream stream;
+				using var httpClient = new HttpClient ();
+
+				try {
+					stream = await httpClient.GetStreamAsync (packageVersionsUrl);
+				} catch {
+					// fallback to embedded resource
+					stream = typeof (ScaffoldingConfig).Assembly.GetManifestResourceStream ("ScaffoldingPackageVersions.json");
+				}
+				
+				using var streamReader = new StreamReader (stream);
+				var json = await streamReader.ReadToEndAsync ();
+				fetchedConfig = JsonConvert.DeserializeObject<ScaffoldingConfig> (json);
+			}
+			return fetchedConfig;
+		}
+	}
+}