feat: NEW PROVIDER: MITTWALD (Mittwald mStudio provider)

[twiesing]

Adds MITTWALD, a DNS provider for mittwald mStudio, built on the official Go client github.com/mittwald/api-client-go.

Please create the GitHub label 'provider-MITTWALD'.

How it maps to mStudio

• mStudio keeps one DNS zone per name (example.com, www.example.com, _dmarc.example.com are separate zones), each with one record set per type: A and AAAA together, CNAME, MX, TXT, SRV, CAA. Corrections are therefore computed with diff2.ByLabel; a name's zone is created when its first record is added and deleted when its last record is removed.
• Record sets that mStudio manages (addresses of a name connected to an ingress, mittwald's mail exchangers) are neither returned nor touched. Declaring A/AAAA/MX records on such a name replaces the managed set.
• auditrecords.go rejects what the API does not accept: wildcard names, NS, null MX, MX preference above 100, empty TXT, TXT with trailing space, CAA values other than a host name, TTLs outside 60..86400, and more records per set than the API allows.

API behaviour handled in the transport (retry.go)

• Rate limit: 3000 requests per 600 s per user. The provider reads X-Ratelimit-Remaining/X-Ratelimit-Reset and waits for the reset once the limit is used up; a 429 (which carries no reset information) is repeated with backoff.
• Eventual consistency: writes return an event ID (ETag); every following request sends it as If-Event-Reached, so reads see the write and writes to a zone just created find it. 412 is repeated; after a write, 403/404 are repeated a few times, as mittwald's own client does.
• 5xx are repeated for requests that cannot create anything twice (never for POST).
• The generated client decodes an unset record set ({}) into several oneOf alternatives at once; sets are only read when they are not unset (regression test included).

Tests

• Integration tests (go test ./integrationTest -args -profile MITTWALD): 217 passed, run several times.
• Unit tests for conversion, audit rules and the transport; provider conversion golden files recorded from a passing integration run. Zone IDs and the test domain in the recordings were replaced with neutral values (00000000-…, mittwald-test.example); the replay does not depend on them.
• go vet, golangci-lint (0 issues), go generate, go mod tidy.

I'll maintain the provider (@twiesing).

[TomOnTime]

Thank you for contributing this new provider, @twiesing !

A few action items before we can merge this PR:

1. By now you should have received a Github invite to join the Provider-maintainers github team, which gives you the "triage" role for this repo. Please accept the invite so we can assign bugs to you.
2. @fm: Faisal Misle is our “liaison to maintainers”. Please send him an email from your preferred address to dnscontrol so we can stay in touch. Your email address will not be shared, only used for DNSControl communication. Please email f at faisal dot fm and CC tal at what exit dot org
3. Please consider setting up a test account for use by our automated testing. This will mean we can test the provider every release in an automated manner. Info is here: https://docs.dnscontrol.org/developer-info/byo-secrets#donate-secrets-to-the-project (If a test account or OE&T environment isn't available, an API key restricted to a single domain is sufficient.)

When I see the Github invite accepted and receive the email, I’ll merge this PR.

Thanks again!
Tom

[TomOnTime]

Looks good! Thanks for submitting this!

[TomOnTime]

Please rebase.

[twiesing]

Rebased onto main.

[TomOnTime]

CC @cafferata The list of providers in README.md is difficult to update and causes a lot of branch conflicts. Can we generate that list instead?

[twiesing]

Rebased again @TomOnTime 😄 🚀

[TomOnTime]

Welcome to the team!