add GHEDomain in TargetCreateParam

[camonster]

Prevent an error even if the user specifies ghe_domain when registering a target

• user command example

  $ curl -XPOST https://${myshoes_shot} -d '{"scope": "xxxx", "ghe_domain": "", "resource_type": "large",  "runner_user": "runner", "runner_version": "v2.302.1"}'
  {"error":"json decode error"}

• myshoes error log

  myshoes 2023/03/08 05:24:22 failed to decode request body: json: cannot unmarshal string into Go struct field TargetCreateParam.ghe_domain of type sql.NullString