OpenPGP for XMPP Pubsub (https://xmpp.org/extensions/inbox/pubsub-encryption.html,
currently a protoXEP) is implemented and activated when `encrypted` is set to `True` in
pubsub's `extra` data.
On item retrieval, the decryption is transparent if the key is known, except if the
`decrypt` key in `extra` is set to `False` (notably useful when one wants to checks that
data is well encrypted).
Methods and corresponding bridge methods have been implemented to manage shared secrets
(to share, revoke or rotate the secrets).
plugin XEP-0060's `XEP-0060_publish` trigger point as been move before actual publish so
item can be modified (here e2ee) by the triggers. A new `XEP-0060_items` trigger point has
also been added.
`encrypted` flag can be used with plugin XEP-0277's microblog data
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