feat: makes subgraph routing-url optional

[EverlastingBugstopper]

fixes #169 it really is this simple!

• If --routing-url is not passed and there is already a routing url for a managed service, the API should leave it untouched
• If --routing-url is not passed and this is the first time they're pushing a service, we should error and tell them they need to pass the --routing-url flag
  • the error message could be a bit better here, currently it's error: URL must be provided when upserting a new service. something better might be error: You must provide a --routing-url when publishing a subgraph for the first time
• If --routing-url is passed and it's the first time pushing a service, it should push the service and exit successfully.

[JakeDawkins]

😂 you really do love to see it

[lrlna]

I am wondering if we should include notes in our docs about these routing-url specifications. What do you all think?

[EverlastingBugstopper]

Probably not a bad idea!

TODO:

• add notes about upserting routing url in help-text and docs
• add changelog entry