Add DNS provider for United-Domains

[ldez]

Closes #2730

Ping @IONOS-Haefele, can you run the command (with your domain, email, credentials, etc.)?

make build
rm -rf .lego

UNITEDDOMAINS_API_KEY="xxx" \
lego --email [email protected] --dns uniteddomains -d '*.example.com' -d example.com -s https://acme-staging-v02.api.letsencrypt.org/directory run

Note that the wildcard domain is important.

How to test this PR?

1. You need Go
2. Check out the PR:

   git clone https://github.com/ldez/lego.git
   cd lego
   git checkout feat/dns/uniteddomains

3. Compile lego:
   • if you have make: make build
   • if you don't have make: go build -o dist/lego ./cmd/lego
4. Run the following command with your information (email, domain, credentials):

   UNITEDDOMAINS_API_KEY="xxx" \
   ./dist/lego --email [email protected] --dns uniteddomains -d '*.example.com' -d example.com  -s https://acme-staging-v02.api.letsencrypt.org/directory run

   The wildcard domain is important
5. Before each run of the command, you should clean your local environment:

   rm -rf .lego