Enable reading cli-input-json from stdin

**Is your feature request related to a problem? Please describe.**
I want to use aws-cli in a pipeline scenario. Therefore I want to pass as argument for `--cli-input-json` the value from stdin.

**Describe the solution you'd like**
The character `-` should serve as indicator, that the value for `--cli-input-json` should be read from stdin.

``` bash
cat sample.json | aws secretsmanager create-secret --cli-input-json -
```

**Describe alternatives you've considered**
No workaround available as far as I know.

**Additional context**
-