Add DNS provider for Mittwald

[ldez]

• [x] adds a description to your PR
• [x] have a homogeneous design with the other providers
• [x] add tests (units)
• [x] add tests ("live")
• [x] add a provider descriptor
• [ ] generate CLI help, documentation, and readme.

• [ ] be able to do: (and put the output of this command to a comment)

  make build
  rm -rf .lego
  
  MITTWALD_TOKEN=yyyy \
  ./dist/lego -m your@email.com --dns mittwald -d *.example.com -d example.com -s https://acme-staging-v02.api.letsencrypt.org/directory run

  Note the wildcard domain is important.

• [x] pass the linter
• [x] do go mod tidy

Ping @jotimann, can you run the command (with your domain, email, and credentials)? I will also ask you to test with a subdomain (foo.example.com). Note:

• the implementation is only based on the API doc (i.e. not tested with real data)
• I still have some questions (FIXME) so I guess this will not work on the first run.

Closes #2177

[ldez]

@jotimann do you need help to build or test the PR?

[jotimann]

@jotimann do you need help to build or test the PR?

Hey @ldez a colleague of mine used lego and requested to add mittwald as a provider. He is currently on vacation and will be back at 08.07. and will test the integration. Hopefully thats okay