os-bind: Add include directory for custom options (named.conf)

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Is your feature request related to a problem? Please describe.
I am trying to configure DoT for connection to forwarders. Since this requires more than just an IP address the current form does not seem to suit. I think adding this functionality to the form may not be the best and so ideally would like to have a separate include directive for custom options.

Describe the solution you'd like
Can we add an include line in the options stanza like #4773, but to allow custom options.
A new directory would also be needed, maybe named-options.conf.d

Describe alternatives you've considered
I have currently edited the named.conf to add this include directive manually, but I assume this will get wiped out with the next upgrade. We could also add this option to the input form, however I think this is less work, is more flexible and also solves other issues like #1715.

I can put together a pull request for this if thats OK?

Thanks again for an awesome firewall!

Locate the os-bind code that generates named.conf and compare its include handling with the approach discussed in issue #4773. Add support for an options include directory such as named-options.conf.d, then verify that custom option files are included in generated configuration and survive upgrades.