fix(lambda): Add 'provided' runtime (#1764)

Provided runtime can be used for custom runtimes.

Fixes #1761.

Author: rix0rrr

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

@@ -12,7 +12,8 @@ export enum RuntimeFamily {
   Python,
   DotNetCore,
   Go,
-  Ruby
+  Ruby,
+  Other
 }
 
 /**
@@ -38,6 +39,7 @@ export class Runtime {
   public static readonly DotNetCore21 = new Runtime('dotnetcore2.1',  RuntimeFamily.DotNetCore);
   public static readonly Go1x =         new Runtime('go1.x',          RuntimeFamily.Go);
   public static readonly Ruby25 =       new Runtime('ruby2.5',        RuntimeFamily.Ruby, { supportsInlineCode: true });
+  public static readonly Provided =     new Runtime('provided',       RuntimeFamily.Other);
 
   /**
    * The name of this runtime, as expected by the Lambda resource.

packages/@aws-cdk/aws-lambda/test/test.lambda.ts

@@ -1286,6 +1286,15 @@ export = {
 
     // THEN
     test.deepEqual(bindCount, 2);
+    test.done();
+  },
+
+  'Provided Runtime returns the right values'(test: Test) {
+    const rt = lambda.Runtime.Provided;
+
+    test.equal(rt.name, 'provided');
+    test.equal(rt.supportsInlineCode, false);
+
     test.done();
   }
 };