Add support for secure Mattermost notifications

[sochotnicky]

Apprise supports this but we need to surface the configuration part and pass it on. Keeping default to insecure connection to not break compatibility, but perhaps worth reconsidering.

Checklist

• This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
  • I think this is small enough of a fix?
• If this pull request adds new functionality, it includes unit tests that cover the changes
• If this pull request removes docs files, it includes redirect settings in mint.json.
• If this pull request adds functions or classes, it includes helpful docstrings.

[codspeed-hq]

CodSpeed Performance Report

Merging #16884 will not alter performance

_{Comparing sochotnicky:add-mattermost-secure-support (719aebf) with main (c2e0a1e)}

Summary

✅ 2 untouched benchmarks

[desertaxle]

Thank you!