Generate a service name when CreateService receives none

• related to: https://github.com/moby/moby/pull/53468#discussion_r3857434198

Consider allowing to create a service without a name and generate one in SwarmKit.

Context: Moby currently generates a random service name in ServiceSpecToGRPC:

https://github.com/moby/moby/blob/466f2d2e5d630e51faf574a49e10d4787e3fa099/daemon/cluster/convert/service.go#L168-L182

This is a bit unexpected for a conversion function, and the name is generated only once. If it already exists, SwarmKit returns:

rpc error: code = AlreadyExists desc = name conflicts with an existing object

Moby now needs to detect this error, generate another name and retry the whole CreateService request:

• https://github.com/moby/moby/pull/53468

I think it would make more sense for SwarmKit to handle this. It owns the service store and already generates the service ID, so it can assign a unique name as part of creating the service.

We probably don't need to duplicate Moby's friendly names generator. Using the generated service ID, or another GUID-like value, should be enough.

This would also let Moby remove service-name generation from the conversion code and avoid every SwarmKit client having to implement the same retry logic.

Start at SwarmKit's CreateService entry point and service store, then review the linked Moby ServiceSpecToGRPC code and PR #53468 for the current name generation and retry behavior. Done means CreateService accepts a missing name, assigns a unique generated name during creation, and allows Moby to remove its duplicate generation and retry logic.