entgql/templates: ability to add extra fields and methods to mutation inputs (#608)

* ability to add additonal fields to mutation input

* Update mutation_input.tmpl removed the model keyword

Author: khanakia

entgql/template/mutation_input.tmpl

@@ -55,6 +55,12 @@ import (
                 {{- end }}
             {{- end }}
         {{- end }}
+
+        {{- with $tmpls := matchTemplate "helper/gql_mutation_input/fields/*"  }}
+            {{- range $tmpl := $tmpls }}
+                {{- xtemplate $tmpl $n }}
+            {{- end }}
+        {{- end }}
     }
 
     // Mutate applies the {{ $input }} on the {{ $n.MutationName }} builder.
@@ -110,6 +116,12 @@ import (
                 {{- end }}
             {{- end }}
         {{- end }}
+
+        {{- with $tmpls := matchTemplate "helper/gql_mutation_input/mutate/*"  }}
+            {{- range $tmpl := $tmpls }}
+                {{- xtemplate $tmpl $n }}
+            {{- end }}
+        {{- end }}
     }
 
     {{- range $b := $n.Builders }}