feat(lambda): .NET Core 3.1 runtime

Fixes #7104

Author: michaeldimoudis

packages/@aws-cdk/aws-lambda/lib/runtime.ts

@@ -46,6 +46,7 @@ export class Runtime {
   /** @deprecated Use {@link DotNetCore21} */
   public static readonly DOTNET_CORE_2 =  new Runtime('dotnetcore2.0',  RuntimeFamily.DOTNET_CORE);
   public static readonly DOTNET_CORE_2_1 = new Runtime('dotnetcore2.1',  RuntimeFamily.DOTNET_CORE);
+  public static readonly DOTNET_CORE_3_1 = new Runtime('dotnetcore3.1',  RuntimeFamily.DOTNET_CORE);
   public static readonly GO_1_X =         new Runtime('go1.x',          RuntimeFamily.GO);
   public static readonly RUBY_2_5 =       new Runtime('ruby2.5',        RuntimeFamily.RUBY);
   public static readonly PROVIDED =     new Runtime('provided',       RuntimeFamily.OTHER);

packages/@aws-cdk/aws-lambda/package.json

@@ -141,6 +141,7 @@
       "docs-public-apis:@aws-cdk/aws-lambda.Runtime.DOTNET_CORE_1",
       "docs-public-apis:@aws-cdk/aws-lambda.Runtime.DOTNET_CORE_2",
       "docs-public-apis:@aws-cdk/aws-lambda.Runtime.DOTNET_CORE_2_1",
+      "docs-public-apis:@aws-cdk/aws-lambda.Runtime.DOTNET_CORE_3_1",
       "docs-public-apis:@aws-cdk/aws-lambda.Runtime.GO_1_X",
       "docs-public-apis:@aws-cdk/aws-lambda.Runtime.JAVA_8",
       "docs-public-apis:@aws-cdk/aws-lambda.Runtime.JAVA_11",