Feature request: sam generate lambda - Adds a lambda function to existing sam template

### Describe your idea/feature/enhancement

I wish SAM CLI would have a sub-command that generates artifacts and updates the sam template. Similar to the `ionic generate` command (https://ionicframework.com/docs/cli/commands/generate)
e.g.
- `sam generate lambda FunctionName` - Allows us to add a lambda function to a SAM template and potentially attach it to API GW
- `sam generate api APIGWName ResourceName LambdaFunctionName` - Allows us to a lambda event and attach it to API GW at the right path

### Proposal

As described above, the SAM CLI would be modified to generate simple lambda functions and update the sam template as well.