Make GraphQL mutation ExternalStatusCheckCreate

What does this MR do and why?

It adds the BranchRules::ExternalStatusChecks::CreateService service and the externalStatusCheckCreate GraphQL node to create .

GraphQL query

mutation TestMutation($branchRuleId: ProjectsBranchRuleID!, $name: String!, $externalUrl: String!) {
  branchRulesExternalStatusCheckCreate(input: {
    branchRuleId: $branchRuleId
    name: $name
    externalUrl: $externalUrl
  }) {
    externalStatusCheck {
      id
      name
      externalUrl
    }
    errors
  }
}

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #439398 (closed)