Eliminate SystemId type, replace with ObjectId

[chrismarget-j]

The SystemId type came from an early observation that seemingly all Apstra objects had UUID4 style IDs. Except for the system_id value used to represent switches. Are they different?

They seem to be not different. Objects in Apstra, whether they're global catalog items (where I'd first encountered the UUID4 scheme), graph nodes and relationships and other objects can use seemingly any string with a surprising variety of permitted characters, including some punctuation.

system_id are not special. They should be lumped in with the rest.

This will be a breaking change.