sam cli verb to list out functions/targets to be built

### Describe your idea/feature/enhancement
I wish SAM CLI would have a command/verb like `sam list` or `sam local list` or `sam local functions` that would parse directories/functions and list out what is going to be built by `sam build`

### Proposal

whatever part of sam build does, that finds targets in the base directory to build, have that as a separate cli action to be able to see what would be built.

maybe a --dry-run flag for build too

### Additional Details
https://github.com/awslabs/aws-sam-cli/blob/af124bc559847f1f687538da7fbc9925783d09f8/samcli/commands/build/build_context.py#L140-L154