Use the Go generated file convention.

This commit updates the template to include the recommended convention
for generated files, specifically a string that matches the regex:

```
^// Code generated .* DO NOT EDIT\.$
```

This change should make easier certain housekeeping tasks, specifically
allowing things like linting tools to ignore these files. See
https://pkg.go.dev/cmd/go#hdr-Generate_Go_files_by_processing_source for
a canonical source for this recommendation.

Author: smulube

cmd/generate-fix/internal/templates.go

@@ -171,6 +171,7 @@ func ({{ template "receiver" }} {{ .Name}}RepeatingGroup) Get(i int) {{ .Name }}
 
 	HeaderTemplate = template.Must(template.Must(baseTemplate.Clone()).Parse(`
 {{ define "receiver" }}h{{ end }}
+// Code generated by quickfix. DO NOT EDIT.
 package {{ .Package }}
 
 import (
@@ -212,6 +213,7 @@ func NewHeader(header *quickfix.Header) (h Header) {
 
 	TrailerTemplate = template.Must(template.Must(baseTemplate.Clone()).Parse(`
 {{ define "receiver" }}t{{ end }}
+// Code generated by quickfix. DO NOT EDIT.
 package {{ .Package }}
 
 import (
@@ -246,6 +248,7 @@ type Trailer struct {
 
 	MessageTemplate = template.Must(baseTemplate.Parse(`
 {{ define "receiver" }}m{{ end }}
+// Code generated by quickfix. DO NOT EDIT.
 package {{ .Package }}
 
 import (
@@ -325,6 +328,7 @@ func Route(router RouteOut) (string, string, quickfix.MessageRoute) {
 	`))
 
 	TagTemplate = template.Must(template.New("Tag").Parse(`
+// Code generated by quickfix. DO NOT EDIT.
 package tag
 import "github.com/quickfixgo/quickfix"
 
@@ -336,6 +340,7 @@ const (
 	`))
 
 	FieldTemplate = template.Must(template.New("Field").Funcs(tmplFuncs).Parse(`
+// Code generated by quickfix. DO NOT EDIT.
 package field
 import (
 	{{ if checkIfTimeImportRequiredForFields . }}"time"{{ end }}
@@ -415,6 +420,7 @@ func (f {{ .Name }}Field) Value() ({{ quickfixValueType $base_type }}) {
 `))
 
 	EnumTemplate = template.Must(template.New("Enum").Parse(`
+// Code generated by quickfix. DO NOT EDIT.
 package enum
 {{ range $ft := . }}
 {{ if $ft.Enums }}