Feature Request: Use codegen in headless mode seperate from push

**Is your feature request related to a problem? Please describe.**
When setting up the build configuration for multiple frontend environments with a shared backend, I have the need to make the build process use ```amplify pull``` to get the most up-to-date metadata to connect to the backend (e.x. the ```aws-exports.js``` file for a React webapp). However, to use any of the GraphQL statements generated by ```amplify codegen```, it seems like I need to store a copy of the statements in each one of the frontend repositories in order to use them. It would be preferable if I could generate the statements during/after pulling to reduce manually duplicating the same files across multiple repositories.

**Describe the solution you'd like**
I think either one of these proposed solutions would work, though the latter would most likely be easier to implement.
- Give an option when running ```amplify pull``` to generate the GraphQL statements if a GraphQL API is detected in the backend (possibly only on the first pull). Something like
```
The current backend environment has a GraphQL API. Would you like to generate new GraphQl statements? (Y/n)
```
Along with the other prompts which appear when running ```amplify codegen```. For headless mode, it would work similarly to how it does for ```amplify push``` by providing a ```--codegen``` flag to the command.
- Give ```amplify codegen add``` a headless mode, similar to that suggested in aws-amplify/amplify-cli#2005 and aws-amplify/amplify-cli#852. This would allow for automated statement generation if I use ```amplify pull``` to get the entire backend definition, which would include the schema. Alternatively, I could just include the schema file in each repository, which would at least minimize the number of files that need to be manually copied and maintained across each one.

**Describe alternatives you've considered**
- Using ```amplify pull``` to get the entire backend definition and then using ```amplify push --codegen``` in headless mode. This doesn't work in this case since just pulling the backend means there are no changes and thus ```amplify push``` doesn't run. Even if it did, it wouldn't be ideal to allow the build process to potentially change the backend since it doesn't need to do so.