chore(integration): read namespace ID from creation / update request, not object

[jvallesm]

Because

• Namespace ID in connection requests was being read from the nested
  object, which caused conflicts in the OpenAPI definitions.

This commit

• Reads the namespace ID as an explicit parameter. This field in the
  Connection object becomes, then, an OUTPUT_ONLY field.

[linear]

INS-6510 Merge all public endpoints into single OpenAPI file