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

[austinvalle]

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