add `--manifest` flag to `copilot init` to allow initializing a service using an existing manifest.yml

[bvtujo]

copilot svc init is currently overloaded, as it's the only way to initialize a service, either from scratch or from an existing manifest. We should allow customers with existing manifests to specify the location of a manifest.yml file and use that file to recreate the input parameters for svc init.

$ copilot svc init --manifest copilot/fe/manifest.yml
Ok, we'll create a Load Balanced Web Service named fe listening on port 8080 using the Dockerfile located at ./path/to/Dockerfile.

Or using an image and a scheduled job:

$ copilot init --manifest copilot/job/manifest.yml --deploy
Ok, we'll create a Scheduled Job named job which will run every 4 hours using the image located at amazon/amazon-ecs-sample.
Creating ECR Repositories...
Deploying...

We should error out on any other flag input.

$ copilot svc init --manifest copilot/fe/manifest.yml --name backend
✘ `--manifest` cannot be specified with any other flags

[huanjani]

We can follow the same workflow to enable multiple pipelines and their corresponding pipeline manifests!

[kohidave]

Maybe we can use the '--manifest' path instead - I'm keen on reserving import for importing existing AWS resources or services.

[huanjani]

Related: #1381

[huanjani]

Resolved with #3715.

[Lou1415926]

Reopening because we are missing the flag in init or deploy!