Allow to pass block to graphql_mutation helper

Adds a syntactic sugar for the `graphql_mutation` method
which allows passing a block as a GraphQL query.

Example before:

```
graphql_mutation(:update_alert_status, variables.merge(input),
                 <<~QL
                   clientMutationId
                   errors
                   alert {
                     iid
                     status
                   }
                 QL
)
```

Example after:

```
graphql_mutation(:update_alert_status, variables.merge(input)) do
  <<~QL
     clientMutationId
     errors
     alert {
       iid
       status
     }
  QL
end
```

spec/support/helpers/graphql_helpers.rb

@@ -77,10 +77,12 @@ module GraphqlHelpers
     QUERY
   end
 
-  def graphql_mutation(name, input, fields = nil)
+  def graphql_mutation(name, input, fields = nil, &block)
     mutation_name = GraphqlHelpers.fieldnamerize(name)
     input_variable_name = "$#{input_variable_name_for_mutation(name)}"
     mutation_field = GitlabSchema.mutation.fields[mutation_name]
+
+    fields = yield if block_given?
     fields ||= all_graphql_fields_for(mutation_field.type.to_graphql)
 
     query = <<~MUTATION