feat: add missing flags for serverless/template management

[TimPietruskyRunPod]

Problem

Several critical serverless and template management operations are missing from the CLI, forcing users to fall back to the web UI or raw GraphQL API calls.

Missing flags

runpodctl serverless create

• --network-volume-id — no way to attach a network volume when creating an endpoint

runpodctl serverless update

• --template-id — can't change the template on an existing endpoint

runpodctl template create

• --volume-mount-path is accepted but ignored (always returns /workspace)

runpodctl template update

• --container-disk-in-gb — can't update container disk size on an existing template

Impact

When deploying a custom Docker image to serverless (e.g. a ComfyUI worker that needs a network volume for models and >5GB container disk), none of these operations can be done end-to-end via the CLI. Users must switch to the web UI mid-workflow.

Expected behavior

All CRUD operations available in the web UI should be available via runpodctl.

[Sanskarzz]

I would like to work on this.