Smooth handling of STDIN when `--input` field is not provided

## Steps to recreate
1. `make build`
2. `mkdir output`
3. Run `./terraform-plugin-codegen-framework generate all`

I was expecting an error that no `--input` path was provided and no path provided on STDIN, instead the program will wait for input (I'm also not able to provide it input after, not sure why that is). I think there might be another bug I'm running into after splitting the commands in #18, but I ran into this issue first.

Possible idea using `Scan`: https://stackoverflow.com/a/49704981