Add mailcow_relayhost resource

[j6s]

Adds a new mailcow_relayhost resource in order to manage relayhosts / sender dependent transport in mailcow.

resource mailcow_relayhost "test_relayhost" {
  hostname = "prod-mta-06.swg-srv.net:2525"
  username = "MY_SWEEGO_SMTP_USERNAME"
  password = "MY_SWEEGO_SMTP_PASSWORD"
}

I will create a second PR that allows setting a relayhost for domains and mailboxes, which is ultimately what I am after.

NOTE: I currently only have my production mailcow instance, so I did not feel entirely comfortable running the tests, which, as I understand, modify the state of my instance. I did, however, verify creating, updating and importing of the resource.

[l-with]

Dear @j6s, thank you very much for your pull request. I like to inform you that the values for secrets I use for the tests in
.github/workflows/go.yml, e.g.
MAILCOW_HOST_NAME, MAILCOW_API_KEY,
are from the official mailcow test instance, as documented in the last line of the README.md.
The test seldom fail, when the mailcow test instance is not available.
You can also use this instance for testing.

[l-with]

The test cannot be executed because of build errors, please fix them and rund the test as described in the README.md