[FEATURE REQUEST] acme: Support manual plugin

[leeclemens]

Is your feature request related to a problem? Please describe.
The acme module/state does not support the manual plugin.

Describe the solution you'd like
Support the manual plugin. This can also facilitate a workaround for other DNS plugins until they are supported.

Describe alternatives you've considered
N/A

Additional context
In my case, I use nsupdate and even certbot does not have a corresponding DNS plugin. The manual plugin is often used for renewing a certificate from a standalone server (which the DNS plugin is very useful for).

Please Note
If this feature request would be considered a substantial change or addition, this should go through a SEP process here https://github.com/saltstack/salt-enhancement-proposals, instead of a feature request.