Feature request: allow to choose one authorizer when there's more than one

### Describe your idea/feature/enhancement

There are APIs where more than one Authorizer may be used, for example an api key + Authorization header.
In those cases `sam local start-api` fails with:

```
Error: There must only be a single authorizer defined for path='/' method='get', found '2'
```

### Proposal

Add some flag that allows the user to choose one instead of bypassing the authorizer:

```
sam local start-api --authorizer AzureAuthorizer
```