no txt records created for root domains because of the recordtype prefix

What happened:

I have seen in my hetzner setup that the txt records for the @.example.com A records won't be created. This leeds also to missing updates when the IP of the lb changes

What you expected to happen:

I did not see an error message but this might be a problem of the webhook provider. When i try to create an TXT entry manually like [email protected] the hetzner api will respond with invalid syntax.

I guess the bug emerged with the new format introduced here https://github.com/kubernetes-sigs/external-dns/blob/0eadcb0ef9e229aa44598f8eeb7aa0f5b260d0af/docs/registry/txt.md?plain=1#L12

it would probably makes sense to add a option like this https://github.com/kubernetes-sigs/external-dns/blob/0eadcb0ef9e229aa44598f8eeb7aa0f5b260d0af/registry/mapper/mapper.go#L96

the option name might be like --txt-root-replacement

How to reproduce it (as minimally and precisely as possible):

you can reproduce it with hetzner provider creating a records at root (@) level

Anything else we need to know?:

Environment:

• External-DNS version (use external-dns --version): 0.18.0
• DNS provider: https://github.com/mconfalonieri/external-dns-hetzner-webhook
• Others: