feat(notifier): actually send messages (part 4 of shoutrrr support)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 76.47059% with 4 lines in your changes missing coverage. Please review.

Project coverage is 90.90%. Comparing base (49cfd68) to head (14be06d).

Files	Patch %	Lines
cmd/ddns/ddns.go	0.00%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #768 +/- ## ========================================== - Coverage 90.95% 90.90% -0.06% ========================================== Files 51 52 +1 Lines 2112 2122 +10 ========================================== + Hits 1921 1929 +8 - Misses 180 182 +2 Partials 11 11 ``` | [Flag](https://app.codecov.io/gh/favonia/cloudflare-ddns/pull/768/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/768/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=favonia) | `90.90% <76.47%> (-0.06%)` | :arrow_down: | 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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