add ipv6 support to the API

[BPplays]

this adds IPv6 support to the API with the option to disable IPv4 or IPv6

Checklist:

• I have read the Contributing guidelines.

[oobabooga]

I have made some small changes but can't push to your repository due to

To github.com:BPplays/text-generation-webui.git
 ! [remote rejected]   HEAD -> ipv6_api (permission denied)
error: failed to push some refs to 'github.com:BPplays/text-generation-webui.git'

Could you enable access/editing?

[BPplays]

@oobabooga sure, try now

[oobabooga]

Thanks, it worked now. The only big change I have made was to change the flag to --api-enable-ipv6 and disable IPV6 by default, since IPV6 is infrequently used. The rest were just some minor syntax simplications. Thanks for the PR!