Add more github actions for better testing

[kaleko]

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 makes sure containers build, cdk.yml makes sure cdk-synth etc runs, frontend.yml does linting and building on frontend files, etc.