Add support for StartDeployment Command

Currently the action creates or updates the service accordingly. If support is added for a flag that jsut calls a `StartDeployment` for the use-cases where a user may not want to update any configuration and jiust trigger a new version of their code/image.

Start by tracing the action's existing service create-or-update flow and locate where AWS App Runner commands are invoked. Add a flag path that calls StartDeployment without changing configuration, then verify that both configuration updates and code/image-only redeployments behave as intended.

Written by the indexing model from the issue text.