Expose the HTTP URL as an APIGateway Stage output property

Adds two output properties on APIGateway Stage resources.
* `url` is the full URL to the root of the deployed stage.
* `executionARN` is the arn needed to pass to Lambda to
give the stage permission to invoke a Lambda handler.

lib/aws/idl/apigateway/stage.go

@@ -48,4 +48,8 @@ type Stage struct {
 	CreatedDate string `lumi:"createdDate,out"`
 	// The timestamp when the stage last updated.
 	LastUpdatedDate string `lumi:"lastUpdatedDate,out"`
+	// The URL to invoke the HTTP endpoint for this API stage.
+	URL string `lumi:"url,out"`
+	// The execution ARN needed to pass to Lambda to give this API stage permission.
+	ExecutionARN string `lumi:"executionARN,out"`
 }

lib/aws/pack/apigateway/stage.ts

@@ -21,6 +21,8 @@ export class Stage extends lumi.Resource implements StageArgs {
     public variables?: {[key: string]: string};
     public createdDate: string;
     public lastUpdatedDate: string;
+    public url: string;
+    public executionARN: string;
 
     constructor(name: string, args: StageArgs) {
         super();

lib/aws/provider/apigateway/stage.go

@@ -106,6 +106,9 @@ func (p *stageProvider) Get(ctx context.Context, id resource.ID) (*apigateway.St
 	}
 	variables := aws.StringValueMap(resp.Variables)
 
+	url := "https://" + restAPIID + ".execute-api." + p.ctx.Region() + ".amazonaws.com/" + stageName
+	executionARN := "arn:aws:execute-api:" + p.ctx.Region() + ":" + p.ctx.AccountID() + ":" + restAPIID + "/" + stageName
+
 	return &apigateway.Stage{
 		RestAPI:             NewRestAPIID(p.ctx.Region(), restAPIID),
 		Deployment:          NewDeploymentID(p.ctx.Region(), restAPIID, aws.StringValue(resp.DeploymentId)),
@@ -116,6 +119,8 @@ func (p *stageProvider) Get(ctx context.Context, id resource.ID) (*apigateway.St
 		Description:         resp.Description,
 		CreatedDate:         resp.CreatedDate.String(),
 		LastUpdatedDate:     resp.LastUpdatedDate.String(),
+		URL:                 url,
+		ExecutionARN:        executionARN,
 	}, nil
 }
 

lib/aws/rpc/apigateway/stage.go

@@ -190,6 +190,8 @@ type Stage struct {
     Variables *map[string]string `lumi:"variables,optional"`
     CreatedDate string `lumi:"createdDate,optional"`
     LastUpdatedDate string `lumi:"lastUpdatedDate,optional"`
+    URL string `lumi:"url,optional"`
+    ExecutionARN string `lumi:"executionARN,optional"`
 }
 
 // Stage's properties have constants to make dealing with diffs and property bags easier.
@@ -206,6 +208,8 @@ const (
     Stage_Variables = "variables"
     Stage_CreatedDate = "createdDate"
     Stage_LastUpdatedDate = "lastUpdatedDate"
+    Stage_URL = "url"
+    Stage_ExecutionARN = "executionARN"
 )