Support E2E DMs on Homeservers with E2E enabled

[sr229]

Describe the bug
Currently on homeservers like nitro.chat (hosted by @Nitrokey) - it's almost impossible to associate your name to NickServ as by design the bot ignores E2E. It would be nice if it compensates for these kinds of Homeservers where E2E is mandated as a mandatory feature.

Expected behavior
DMs to NickServ should work as-is.

Additional context
So far I only tested it against https://nitro.chat, but this may be reproducible on any Synapse servers with E2E enabled by default.

[Half-Shot]

This would basically require us to support e2be (End-to-bridge-encryption), which isn't likely to be completed soon because it's a difficult task (see matrix-org/matrix-appservice-bridge#234). However, I'll post an update in here if we make any progress on that.