mod_pubsub_serverinfo: implemented all basic features This commit replaces the earlier proof-of-concept to a solution that: - reports on remotely-connected domains - uses disco/info to detect if those domains opt-in - publishes domain names for remote domains that do so - caches the disco/info response

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summary: External Service Discovery
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This module implementing [XEP-0215] is included with Prosody 0.12 but made available here for older versions.

Documentation is available on the [Prosody site][doc:modules:mod_external_services].