sam local start-api detached mode

### Describe your idea/feature/enhancement

Allow `sam local start-api` to run detached

### Proposal

While SAM is great for local development it can fall short for CI or even test commands. In order to run tests automatically against my **api-gateway** I need to run `sam local start-api` this command continues running until I press Ctrl-C. This works well for local development but I would like to script this out into a CI that will allow me to run integration tests without deploying to aws.

Things to consider:
1. Will this require any updates to the [SAM Spec](https://github.com/awslabs/serverless-application-model)
- No, as far as I know, this should require any updates to the spec.

2. Additional arguments/commands
- By adding a detached mode that also means there must be a non-interrupt way to stop the api. I suggest `sam local stop-api` as a way to stop the api. This command would be dependent on the directory and template that is running and thus the running instance would be a singleton.
- I would suggest `-d` as this would follow the docker command scheme and be more familiar to developers.