Support the Service L2 construct for App Runner.

• implementation
• unit testing and integ testing
• README

What's included

This PR introduces the initial L2 implementation of the Service construct. We are allowed to create App Runner services with:

1. ECR public
2. ECR(private) from any existing ECR repository
3. ECR(private) built and pushed from local assets
4. remote github repository

Design

AWS App Runner allows us to create Service with ECR_PUBLIC, ECR and Github. We should specify the source with the source property to define the source of the repository for the Service and conditionally create the required IAM access role for ECR to pull the required images(doc).

source: Source.fromEcrPublic()  // To define a source from ECR Public container image.
source: Source.fromEcr()  // To define a source from ECR container image.
source: Source.fromGitHub()  // To define a source from a GitHub repository.
source: Source.fromAsset()  // To define a source from local code asset directory.

The connection for Github is required for Source.fromGitHub(). However, as there's no cloudformation support to create the App Runner connection, an existing connection will be required for service with github as the source.

Closes: #14813

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license