feat(api): make record comment configurable

[favonia]

(Fully) close #715.

BREAKING CHANGE: the record comment was always "Created by cloudflare-ddns", but now the default record comment is empty despite being configurable.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.35%. Comparing base (73ea203) to head (c14c1a8).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #783 +/- ## ========================================== + Coverage 90.32% 90.35% +0.02% ========================================== Files 52 52 Lines 2161 2167 +6 ========================================== + Hits 1952 1958 +6 Misses 198 198 Partials 11 11 ``` | [Flag](https://app.codecov.io/gh/favonia/cloudflare-ddns/pull/783/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=favonia) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/favonia/cloudflare-ddns/pull/783/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=favonia) | `90.35% <100.00%> (+0.02%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=favonia#carryforward-flags-in-the-pull-request-comment) to find out more.

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