What would you like to be added:
Currently, at least for Azure, the TTL is hard coded to 300 seconds. It is possible to override this per endpoint but it would be very useful to set the default ttl on the external-dns service itself.

Why is this needed:
We want to use the concept of "convention over configuration" and since we need to lower the TTL we do not want to explicitly do so in all the ingresses we define, but rather define this centrally for our cluster. The hard coded value of 300 seconds means that clients sometimes cache invalid responses for up to 5 minutes, which is unacceptable in our situation.