NEW PROVIDER: HETZNER DNS Console (#904)

* HETZNER: implement the provider for Hetzner DNS Console

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* HETZNER: apply review feedback

- add domain into error messages
- insert sub-strings using `%q`
- insert sub-errors using `%w`
- change api.getZone() signature to return a (potentially `nil`) Zone
   pointer instead of a (potentially empty) Zone value
- sort imports and confirm with `$ goimports -w providers/hetzner/`
- use exact 'api_key' term in error message of settings validation
- add blank line for logic separation
- drop internal record id from correction messages

Co-Authored-By: Tom Limoncelli <tlimoncelli@stackoverflow.com>
Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* HETZNER: add request rate-limiting handling

There are a limited number of data-points on how their rate-limiting
 works at this time. I deduce from my account to others and use a fixed/
 constant backoff of 1s as the initial delay. Thereafter exponential
 increase with factor 2 (not needed at this time).
Hetzner has not made any official statements on rate-limiting, so this
 is guesswork only.

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* HETZNER: address golint complaints

- baseUrl -> baseURL
- mark Record as private -> record
- mark Zone as private -> zone
- mark RequestRateLimiter as private -> requestRateLimiter
- capitalize Id fields as ID
- keep delay logic on same level, move return out of branch

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* HETZNER: rate_limited: init the response timestamp on requestRateLimiter

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

* HETZNER: requestRateLimiter: align local variable with struct name

Signed-off-by: Jakob Ackermann <das7pad@outlook.com>

Co-authored-by: Tom Limoncelli <tlimoncelli@stackoverflow.com>

integrationTest/integration_test.go

@@ -693,7 +693,7 @@ func makeTests(t *testing.T) []*TestGroup {
 		),
 
 		testgroup("empty TXT",
-			not("CLOUDFLAREAPI", "HEXONET", "INWX", "NETCUP"),
+			not("CLOUDFLAREAPI", "HETZNER", "HEXONET", "INWX", "NETCUP"),
 			tc("TXT with empty str", txt("foo1", "")),
 			// https://github.com/StackExchange/dnscontrol/issues/598
 			// We decided that permitting the TXT target to be an empty