Postfix: add support for -o inet_protocols=ipv4 under just the smtp setting

[marner2]

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Is your feature request related to a problem? Please describe.
I get emails rejected from GMail randomly based on how they are feeling about PTR records with ipv6. However, I would like to continue receiving emails on my ipv6 addresses.

Describe the solution you'd like
I would like to be able to specify the smtp protocol from the user interface. Specifically, the solution to the problem described here on the official postfix documentation: https://www.postfix.org/postconf.5.html#smtp_address_preference

The setting "smtp_address_preference = ipv4" is not a solution for remote servers that flag email received over IPv6 as more 'spammy' (the client IPv6 address has a bad or missing PTR or AAAA record, bad network neighbors, etc.). Instead, configure Postfix to receive mail over both IPv4 and IPv6, and to deliver mail over only IPv4.

/etc/postfix/main.cf:
   inet_protocols= all
/etc/postfix/master.cf
    smtp ...other fields... smtp -o inet_protocols=ipv4

Describe alternatives you've considered
I could edit the /usr/local/postfix/master.cf file directly, but that gets overwritten every time I change any other postfix settings. I could also disable ipv6 entirely, but then mail delivered to my ipv6 addresses would get rejected.

I could also use a separate postfix instance to send mail directly rather than relaying through the firewall, but I prefer to keep all of those settings centralized with the incoming server as well.