plugin XEP-0447: handle attachment and download: - plugin XEP-0447 can now be used in message attachments and to retrieve an attachment - plugin attach: `attachment` being processed is added to `extra` so the handler can inspect it - plugin attach: `size` is added to attachment - plugin download: a whole attachment dict is now used in `download` and `file_download`/`file_download_complete`. `download_uri` can be used as a shortcut when just a URI is used. In addition to URI scheme handler, whole attachment handlers can now be registered with `register_download_handler` - plugin XEP-0363: `file_http_upload` `XEP-0363_upload_size` triggers have been renamed to `XEP-0363_upload_pre_slot` and is now using a dict with arguments, allowing for the size but also the filename to be modified, which is necessary for encryption (filename may be hidden from URL this way). - plugin XEP-0446: fix wrong element name - plugin XEP-0447: source handler can now be registered (`url-data` is registered by default) - plugin XEP-0447: source parsing has been put in a separated `parse_sources_elt` method, as it may be useful to do it independently (notably with XEP-0448) - plugin XEP-0447: parse received message and complete attachments when suitable - plugin XEP-0447: can now be used with message attachments - plugin XEP-0447: can now be used with attachments download - renamed `options` arguments to `extra` for consistency - some style change (progressive move from legacy camelCase to PEP8 snake_case) - some typing rel 379

Those certificates are used to activate TLS for end-2-end testing (to be as
close as possible as production environment), they are used in other containers
needing TLS certificates (notably Prosody).

To generate them, minica has been used. Minica can be found at https://github.com/jsha/minica.

The following commands have been used:

$ minica --domains "server1.test,*.server1.test,server2.test,server3.test,libervia-backend.test,libervia-web.test"
$ chmod 0644 minica.pem server1.test/cert.pem && chmod 0640 server1.test/key.pem

Note that certificates are valid for 2 years and 30 days, so they must be renewed after this delay.