Headless support for amplify add codegen

**Is your feature request related to a problem? Please describe.**
I am trying to add multi env support for externally linked APIs but this is currently not possible.

**Describe the solution you'd like**
Headless support for amplify add codegen that lets you pass the codegen config as an argument.

```
CONFIG="{\
\"generateCode\":true,\
\"codeLanguage\":\"typescript\",\
\"fileNamePattern\":\"src/graphql/**/*.ts\",\
\"generatedFileName\":\"src/API.ts\",\
\"generateDocs\":true\
}"

amplify add codegen --apiId xxxxxxxxxxxxxxxxx --config $CONFIG
```

**Describe alternatives you've considered**
Add multi env support for externally linked APIs that allows you to link each environment to a specific AppSync ID.

**Additional context**