Add DNS provider for EUsrv

[ldez]

Closes #1990

This PR needs to be tested by someone with an Euserv account.

Can you run the command with your domain, email, credentials, etc. (and put the output of this command to a comment)?

make build
rm -rf .lego

EUSERV_EMAIL="user@example.com" \
EUSERV_PASSWORD="xxx" \
EUSERV_ORDER_ID="yyy" \
./dist/lego --dns euserv -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/euserv

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):

   EUSERV_EMAIL="user@example.com" \
   EUSERV_PASSWORD="xxx" \
   EUSERV_ORDER_ID="yyy" \
   ./dist/lego --dns euserv -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