Add more github actions for better testing

As an example, this repo which has some nice GH actions, we might want to draw from this and add some of these to FAST or FAST samples repo: https://github.com/aws-samples/bedrock-chat/actions/

For example, [backend.yml](https://github.com/aws-samples/bedrock-chat/blob/v3/.github/workflows/backend.yml) makes sure containers build, [cdk.yml](https://github.com/aws-samples/bedrock-chat/blob/v3/.github/workflows/cdk.yml) makes sure cdk-synth etc runs, [frontend.yml](https://github.com/aws-samples/bedrock-chat/blob/v3/.github/workflows/frontend.yml) does linting and building on frontend files, etc.