Support S3 as `Source` provider in pipeline

[Lou1415926]

There is only a limited set of providers that CodePipeline support for its "Source" stage. However, folks usually can work around this limitation by hooking an S3 bucket with their code repo using a webhook. This requires the S3 bucket to be the "Source" provider.

Copilot today supports CodeCommit, Bitbucket cloud and GitHub. It'd be great if it supports S3 as well, so that people can connect their own code repo to the Copilot pipelines.

Related: https://app.gitter.im/#/room/#aws_copilot-cli:gitter.im/$T8T0CoxQn23r8CtxzyqRTD9YFFSdVaIYUe1DS7gn66w