Integration with rspamd

[MTRNord]

There are currently seemingly 2 ways of connecting this to rspamd.

One being the milter proto and the other one using the http api. On first sight, the http one seems a lot nicer.

I asked about this in https://matrix.to/#/!tNuTutAsraCZsJFZOS:matrix.org/$kijaM-MAMgzHYcpBBhQwLteRbSFFNu3Om_pZBzt7Sew?via=matrix.org&via=midnightthoughts.space (their irc) what the best option is.

It probably makes sense to do this using some kind of modular trait thingy to be able to integrate alternatives too.

[MTRNord]

We will go with the HTTP api for now

[MTRNord]

This requires #22

[MTRNord]

We now do push to rspamd.

Missing is applying the actions and to make sure that rspamd runs in a queue to make sure that it wont crash the connection if rspamd is missconfigured