Add vultr certbot authenticator plugin

[tcuthbert]

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• I have read the contributing guideline and understand that I have made the correct modifications

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Description:

This change introduces a vultr DNS authentication plugin for certbot.

Benefits of this PR and context:

vultr is a popular VPS provider that supports dynamic dns, this PR may be useful to some of your users.

How Has This Been Tested?

Running the x86_64 build locally on my vultr instance. Confirmed valid LE certificate has been installed in my container.

In addition to this I have successfully built the arm64 image with the latest upstream docker image.

from my vultr instance.

git pull $MY_REPO; cd docker-swag
sudo docker build   --no-cache   --pull   -t ghcr.io/linuxserver/swag:latest .
sudo -u my_user docker run \
        --name=swag \
        --cap-add=NET_ADMIN \
        -e PUID=666 \
        -e PGID=666 \
        -e URL=mydomain.xyz \
        -e SUBDOMAINS=me \
        -e VALIDATION=dns \
        -e DNSPLUGIN=vultr \
        -p 127.0.0.1:443:443 \
        -v /srv/proxy/config:/config \
        ghcr.io/linuxserver/swag
sudo cat /srv/proxy/config/etc/letsencrypt/archive/*.xyz/cert1.pem | openssl x509 -noout -subject
subject=CN = me.mydomain.xyz

Source / References:

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[LinuxServer-CI]

I am a bot, here are the test results for this PR:
https://ci-tests.linuxserver.io/lspipepr/swag/1.14.0-pkg-3de043cc-pr-106/index.html
https://ci-tests.linuxserver.io/lspipepr/swag/1.14.0-pkg-3de043cc-pr-106/shellcheck-result.xml