Surface DeploymentCanarySettings on Serverless::Api

**Description:**

Need to surface the [DeploymentCanarySettings](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-apigateway-deployment-deploymentcanarysettings.html) setting for API Gateway RestApis.

This will allow customers to use these deployment settings to deploy a new api revision gradually.

**Caveats:**

SAM doesn't know about previous deployment resources (ie SAM is stateless, doesn't know about older versions of the same template). If a new deployment is created (changes made to DefinitionBody), it will be impossible in SAM to update these canary settings on an old deployment, which will force users to update those settings via the console or CLI.