API Gateway REST API - Update CORS cli params to override template values, or allow flag for * origin/headers

## Feature Request

Per the docs, for REST API gateway "if the endpoint config has CORS set to true, the plugin will use the CLI CORS options for the associated route. Otherwise, no CORS headers will be added." We have a custom header in our CORS configuration which prevents us from using `cors: true`. In my opinion, if the CLI params `corsAllowHeaders` and `corsAllowOrigin` are provided it should override template values for any endpoint with CORS configuration, not just endpoints with `cors: true`.

If others disagree an additional flag `--corsAllowAll` which sends `*` value for all CORS headers will allow serverless offline to support local development when a team has a custom CORS configuration. Perhaps both of these changes should be made.

**Expected behavior/code**

User values for CLI params `corsAllowHeaders` `corsAllowOrigin` should override template values for all endpoints with a CORS config, and/or when user adds `--corsAllowAll` flag `*` value is used for all CORS headers on all endpoints (Access-Control-Allow-Headers, Access-Control-Allow-Methods, Access-Control-Allow-Origin, etc).