edns behaviour on forward only configuration

Description

I would like is a way to fix the advertised payload and guarantee it will always stayed fixed because I know my environment and I don't need BIND to try and work out what the forwarders or the intervening will permit.

1. Use existing settings to infer how EDNS payload advertisement should work. e.g. if 'forward only' is enabled, don't do slow start. The danger with this is there might be a use case that needs both. or
2. A new configuration parameter e.g. 'edns-udp-behaviour' that takes arguments "fixed" or "auto"

see RT #16201