Allow Policy to be Settable from annotation

[Syndlex]

What would you like to be added:

It would be nice to have the option to set the Policy (sync, upsert-only, create-only) from a annotation.

Why is this needed:

We have some Domains that we want to manage very strictly and some domains that are managed as sync. It would be nice to have the option to set the Policy with a annotation.

[ivankatliarchuk]

You could annotate with any annotation and deploy multiple external dns with different flags and filters. The yse case does not sound practical to have dedicated annotation for that functionality.