security/acme-client: support 2fa for inwx API

[pkoevesdi]

Important notices

• I have read the contributing guide lines at https://github.com/opnsense/plugins/blob/master/CONTRIBUTING.md
• I have searched the existing issues, open and closed, and I'm convinced that mine is new.
• When the request is meant for an existing plugin, I've added its name to the title.

Is your feature request related to a problem? Please describe.
When using inwx as domain host, I cannot set 2fa inside this plugin.

Describe the solution you'd like
An additional field for the shared secret and a functionality of the plugin to gereate the TOTP from it, when requested.

Additional context

[fraenki]

Note to myself: upstream documentation...
https://github.com/acmesh-official/acme.sh/wiki/dnsapi#dns_inwx