First implementation - Githubissues

This PR adds first cert-manager webhook implementation for Exoscale. See README.md for details.
There is no build system yet, Github action will be configured in the separate PR.
DNS01 provider conformance testing suite report:
$  TEST_ZONE_NAME=cert-manager-webhook-test.jns.vc. make test  
TEST_ZONE_NAME=cert-manager-webhook-test.jns.vc. go test -v .
=== RUN   TestRunsSuite
2022/08/24 17:40:39 call function Initialize
=== RUN   TestRunsSuite/Basic
=== RUN   TestRunsSuite/Basic/PresentRecord
    suite.go:37: Calling Present with ChallengeRequest: &v1alpha1.ChallengeRequest{UID:"", Action:"", Type:"", DNSName:"example.com", Key:"123d==", ResourceNamespace:"basic-present-record", ResolvedFQDN:"cert-manager-dns01-tests.cert-manager-webhook-test.jns.vc.", ResolvedZone:"cert-manager-webhook-test.jns.vc.", AllowAmbientCredentials:false, Config:(*v1.JSON)(0xc000393cf8)}
2022/08/24 17:40:40 call function Present: namespace=basic-present-record, zone=cert-manager-webhook-test.jns.vc., fqdn=cert-manager-dns01-tests.cert-manager-webhook-test.jns.vc.
2022/08/24 17:40:40 found client credentials in environment, ignoring config
2022/08/24 17:40:52 call function CleanUp: namespace=basic-present-record, zone=cert-manager-webhook-test.jns.vc., fqdn=cert-manager-dns01-tests.cert-manager-webhook-test.jns.vc.
2022/08/24 17:40:52 found client credentials in environment, ignoring config
2022/08/24 17:40:59 call function CleanUp: namespace=basic-present-record, zone=cert-manager-webhook-test.jns.vc., fqdn=cert-manager-dns01-tests.cert-manager-webhook-test.jns.vc.
2022/08/24 17:40:59 found client credentials in environment, ignoring config
2022/08/24 17:41:00 domain record "cert-manager-dns01-tests" not found, nothing to do
2022/08/24 17:41:05 call function Initialize
=== RUN   TestRunsSuite/Extended
=== RUN   TestRunsSuite/Extended/DeletingOneRecordRetainsOthers
2022/08/24 17:41:06 call function Present: namespace=extended-supports-multiple-same-domain, zone=cert-manager-webhook-test.jns.vc., fqdn=cert-manager-dns01-tests.cert-manager-webhook-test.jns.vc.
2022/08/24 17:41:06 found client credentials in environment, ignoring config
2022/08/24 17:41:11 call function Present: namespace=extended-supports-multiple-same-domain, zone=cert-manager-webhook-test.jns.vc., fqdn=cert-manager-dns01-tests.cert-manager-webhook-test.jns.vc.
2022/08/24 17:41:11 found client credentials in environment, ignoring config
2022/08/24 17:41:54 call function CleanUp: namespace=extended-supports-multiple-same-domain, zone=cert-manager-webhook-test.jns.vc., fqdn=cert-manager-dns01-tests.cert-manager-webhook-test.jns.vc.
2022/08/24 17:41:54 found client credentials in environment, ignoring config
2022/08/24 17:42:02 call function CleanUp: namespace=extended-supports-multiple-same-domain, zone=cert-manager-webhook-test.jns.vc., fqdn=cert-manager-dns01-tests.cert-manager-webhook-test.jns.vc.
2022/08/24 17:42:02 found client credentials in environment, ignoring config
2022/08/24 17:42:03 domain record "cert-manager-dns01-tests" not found, nothing to do
2022/08/24 17:42:03 call function CleanUp: namespace=extended-supports-multiple-same-domain, zone=cert-manager-webhook-test.jns.vc., fqdn=cert-manager-dns01-tests.cert-manager-webhook-test.jns.vc.
2022/08/24 17:42:03 found client credentials in environment, ignoring config
--- PASS: TestRunsSuite (92.11s)
    --- PASS: TestRunsSuite/Basic (21.00s)
        --- PASS: TestRunsSuite/Basic/PresentRecord (21.00s)
    --- PASS: TestRunsSuite/Extended (62.41s)
        --- PASS: TestRunsSuite/Extended/DeletingOneRecordRetainsOthers (62.41s)
PASS
ok      github.com/exoscale/cert-manager-webhook-exoscale       92.134s
exoscale / cert-manager-webhook-exoscale

First implementation #1
