Help with designing headless payloads

**Page**: [`/cli/usage/headless`](https://docs.amplify.aws/cli/usage/headless/#example-amplify-add-api-headless-configuration)

**Feedback**:

It is very difficult to assemble JSON payload for all headless commands for all user options. It would be of a great help to have a command line option to generate a payload. Commands will walkthrough all the user options and only emit the final configuration/payload JSON file that can be then used for executing headless command

For example, user will execute the following command:
amplify add auth --gen-headless-payload > auth-payload.json
amplify add api --gen-headless-payload > api-payload.json

Then these files can be modified and preserved to be used in CI/CD like this:
cat auth-payload.json | jq -c | amplify add auth --headless

Start by reviewing the /cli/usage/headless documentation and the entry points for `amplify add auth` and `amplify add api`. Determine how all user options could be walked through and emitted as JSON, then verify that the generated payloads can be modified and consumed by the existing `--headless` commands.

Written by the indexing model from the issue text.