mod_spam_filter: handle rtbl blocked domains

[sstrigler]

This adds support for RTBL blocked domains similar to how prosody's mod_anti_spam does it, following the guide at https://xmppbl.org/dev. It subscribes to the spam_source_domains pubsub node at the given rtbl_host (eg. use "xmppbl.org") and retrieves a list of domains. Presence and message stanzas from there are treated like block-strangers, means only if there's a roster subscription they are being let through.

I've add the following commands:

• get_blocked_domains
• add_blocked_domain
• add_to_spam_filter_cache

[badlop]

Now that the PR passes the tests, it can be merged whenever you consider it stable enough.

Otherwise, if you still want to make improvements, can you please rebase this branch with recent "master"?

Specifically to get commit 3c10f86: the test action in this repository pulled elixir container images from docker hub, and it seems docker hub now has a pull quota... so I've updated to another container registry.

[sstrigler]

Please merge it then. I’ll create extra PRs for upcoming changes.