add `service update` for changing image, repo, and service config without dashboard

[yigitkonur]

problem

once a service is created, there's no cli command to update its configuration — changing the docker image, attaching/detaching a github repo, renaming, or switching between source/image deployment modes all require the dashboard.

real-world scenario

deploying a litellm proxy: created a service with --image, later needed to switch to source deployment with a dockerfile. the only option was to delete and recreate the service, losing all variable configuration. similarly, attaching a github repo for auto-deploy requires the dashboard.

proposed solution

railway service update <name> --image <new-image>       # change docker image
railway service update <name> --repo <user/repo>        # attach github repo
railway service update <name> --name <new-name>         # rename service
railway service update <name> --no-repo                 # detach repo

related

• Allow to create service from CLI without any input #799 covers creation gaps, this covers post-creation management
• the graphql api supports serviceUpdate and serviceConnect mutations